I came across this page while searching the web, very fun.  Maybe because I can relate to some of them.
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This aptly named page is for all the things that the instruction book didn't tell me about, all the disasters and near misses, in general, the invaluable knowledge that comes from life's little "learning experiences". You know what I mean.

1. Watch out for the tenting on the glide-out dinette (or, if the beds don't slide easily, there has to be a good reason).
I came close to destroying the clear vinyl windows on the glide out when folding the trailer for the first or second time. "Gee, this bed doesn't want to go in, does it?" (PUSH SHOVE JAM PUSH HEAVE HO etc...) - the next time I opened the unit up I noticed grease stains on the glide out canvas, and "distortion" on the vinyl windows - Stupid Pop-Up Camper Trick. Make sure the glide out tenting is pulled away from the beds before sliding them in or out. If the beds don't slide in easily, stop and take a look.

2. That little water connector where the outside shower hose connects to the faucet? It is about as strong as a twig.
The outside shower hose was kinked. Water would not come out. I tried to straighten the hose. *SNAP* - Stupid Pop-Up Camper Trick. Fortunately, it was covered by warranty. The dealer put a better hose on that won't kink.

3. So that's why they tell you to chock the front and back of the trailer wheels, even if you're on an incline...
I was setting the unit up in front of the house so we could work on stocking it. I use a wood block under the wheel closest to the curb to compensate for the "crown" in the road - it makes the trailer level. The road in front of the house is a gentle hill, but the trailer can be leveled with the tongue jack. No problem. With the hill, I really only need to chock the downhill side of the trailer wheels, right? It's not going to roll UPHILL, right? Wrong. As soon as I raised the coupler off of the ball, the trailer lurched forward, uphill, impaling the coupler into the plastic bumper of our minivan. The problem? The curb wheel was on the block, but too close to the front of the block. It rolled right off as soon as the coupler was free. Stupid Pop-Up Camper Trick.
 
4. The shower curtain that comes with your unit (if you have a shower) has the structural integrity of a piece of tissue.
Ours had several rips and tears after setting it up the first time. Plan on replacing yours with a more durable, heavy vinyl curtain. We now have a clear shower curtain, and the shower feels less cramped to us than it did with Coleman's translucent curtain.
 
5. Twenty gallons of water isn't much.
Perhaps you are thinking about purchasing a unit with a shower, and are wondering how long the on-board water will last. Our unit has a 20 gallon tank. I found that a typical "nautical" shower (get wet, soap and shampoo, rinse off) used about half of the tank. I found out the hard way, the water ran out in the middle of my wife's shower the next morning. Stupid Pop-Up Camper Trick. If you are hooked up to a campground water supply, no problem, but if you are running off the tank, you will need frequent refills if you use the shower. For use without the shower, I would expect the 20 gallons to last for a weekend trip if you take steps to conserve the water. The good news - the built-in shower is a real treat. Some campground showers can be crowded or messy (or both). Having one on the camper is really nice.
 
6. Inspect your plumbing for leaks the first time you use it.
In fact, it doesn't hurt to check briefly every time you set the unit up, to make sure that something hasn't bounced loose. We discovered a stubborn leak at the galley sink trap on our first trip and set up a pan to catch the water before it could do any damage to the cabinet.
 
7. The water pump is running but nobody is using water...
One night, I heard the pump motor running periodically during a time when none of the water fixtures were in use. Once one realizes that the pump should only run when water is being used, one gets an "anxious feeling". A quick inspection outside revealed water gushing from the area of the hot water heater. The hot water heater has a safety valve that is designed to open under high water temperature or pressure situations. The one on my hot water heater had opened, but there was not a problem with excessive temperature or pressure. When I returned to the dealer for warranty service, their tests revealed that the safety valve was opening when the water temperature reached 90 degrees F. Stupid Pop-Up Camper Hot Water Heater Trick!
 
8. Unburned LP gas is explosive! Big time!
At an open beach campsite, the sea breeze was particularly strong one night. Strong enough to create a back draft in the water heater burner sufficient enough to snuff out the flame. This happened several times after I first lit the water heater. Each time, I would turn the gas off, wait the requisite five minutes for the remaining LP gas to dissipate, then re-light the burner. Well, almost each time. Once, I didn't wait, nor did I turn off the gas, inadvertently I might add. I stuck my long-stem BIC lighter in the hole and pulled the trigger, and faWOOOM!  Fortunately, both I and the camper are still in one piece, except for a few singed hairs on my right hand and eyebrows. It could have been worse. I can safely say that this is the kind of mistake one only needs to make once. The moral of this story is Propane Is Mighty Dangerous Stuff. Be careful.
 
9. Folding trailers are not designed for use in gale force winds.
I spent a week with my sons at Cape Hatteras the week before Labor Day, 1998. The remnants of Hurricane Earl paid a visit late Thursday night. The first problem was the awning. As the winds intensified, it became apparent that leaving it up would be risky. I asked my 10 year old son for assistance with taking it down. We got it lowered just fine, and just as it seemed like I would be able to get it rolled up and stowed, a heavy gust grabbed the awning and blew it out of my son's hands. Well, that did it for him. He politely excused himself and returned to the inside of the camper. I was left standing there with the wind a-blowin' and the awning a-flyin'. A neighbor spotted my predicament and together we were able to get it stowed safely with no damage to the awning or the trailer. At that point, the winds appeared to be 15-20 MPH sustained with higher gusts. Later in the evening, a series of squalls moved through the area, each one stronger than the previous. We were getting shaken around a bit, but things still seemed under control. I went to bed at around 11:00PM, only to be awakened a few hours later at around 1:00AM. We were in the middle of a Major Gale. The wind was blowing hard enough to physically force the rain through the Sunbrella fabric. Our beds were soaked on the windward side of the camper. The roof was rocking wildly. I began to wonder if we were about to be blown across the campground like a giant tumbleweed. When this storm subsided, I looked around outside and found that my neighbors had folded their unit down and retreated to their minivan. I moved my sons to our minivan and folded our trailer down too. If it's good enough for him, it's good enough for me.
As it turned out, the 1:00AM gale was the last one. At 4:00 AM I set the camper up again and moved us back in. The kids didn't really remember the move the next day. No damage to our camper, or any others in the campground, though all the pop-up owners that I spoke to reported rough nights with lots of water inside. Most reported water blown through fabric walls, zippers, seams and Velcro. Sustained winds of over 50 MPH were recorded during the 1:00 AM gale. Not something I'd want to do again anytime soon.
 

10. Something you will want to try the next time you restock your trailer.
Here's a good one. Try it sometime. I highly recommend it.
Set up your camper in the driveway for pre-trip restocking and cleanup. You're not going to sleep in it, so pull out the bunk ends and stretch the tenting out over them, but don't bother raising the bunk end bar and installing the pole. Just let the tent lay on top of the beds. That way the birds won't be able to crap on your mattresses.

Then, ignore/forget to check the weather forecast and allow it to rain all night on your partially set up camper. A steady, soaking rain is best.

The next day, go on out and have a look. You will find:

Bed ends totally flooded
Mattresses soaked
The "floor" is now a "lake".
You see, the tenting doesn't really help keep the water out unless it's properly set up. Instead, it makes "swimming pools" on top of  your beds. When these "swimming pools" leak through the zippers of the bunk end tenting, the water will go right into your partially set up trailer.
Here are just a few of the things I learned from this experience (which, by the way, I conveniently decided to try three days before our first trip of 1999):

The pre-1999 Coleman bed mattresses are really just big sponges with a fabric covering.
Structurewood holds up very well when exposed to a flood.
It sure is difficult to get the big sponges out of the fabric coverings to let them dry faster.
The camper's furnace makes a pretty good "soaked camper dryer" if you crank it up all the way for a couple of days.
It is VERY difficult to get the big sponges BACK IN the fabric covering after they've dried.
Coleman/Fleetwood makes a pretty good product for it to go through this kind of abuse and not sustain any permanent damage.

11. Folding trailers are not designed to be driven through ditches.
Coming out of the parking lot of a rural gas station, I managed to drive the right side of the trailer through a ditch that was next to the gas station's exit. I watched in the rear view mirror as the wheel fell into this monster and popped back out. Both stabilizer jacks on the right side were mangled, and I lost the sand shoe on one. I managed to straighten out the stabilizers (sort of) and I replaced the missing sand shoe, but they'll never be the same again.
 
12. Make sure the water hookup works before setting up at a site.
Something looked funny about Site 11 at Pocahontas State Park. The fresh red mud all around the base of the water hookup looked out of place, since it had been some time since this area has had any rainfall. But, it was 1:00AM, and it didn't register until we had set up completely and I was hooking up the water hose. I cranked on the spigot and NOTHING. Not a drop. Closer inspection revealed that the water hookup had apparently suffered an underground leak earlier and had been shut off by the park management. Next time I'll do a quick water test if there's any doubt.
 
13. Bye bye battery box lid.
Before the trip I "topped off" the battery with a quick charge from my battery charger. After putting the charger away, I hopped in the van with the family and took off. Upon arrival at our destination, I noticed that I no longer had a battery box lid or strap.
 
14. It helps to remove the wheel chocks before attempting to move the trailer.
We had just finished packing, folding down and hitching up, and it was time to pull the trailer forward to retrieve the Lynx Levelers. I asked my wife to get in the van and pull forward two feet (forgetting that the chocks were still in place). She did (with some difficulty). *CRUNCH*

Source: http://www.popupcamping.com/stupid.html