I don't ever use a gear list, I just sift through the gear closet for whatever gear I think I need for a given trip.
Hence, with somewhat regularity I find that I forget something important.
Off the top of my head, it seems that a head lamp and my spoon are the two most common items for me to forget. I've also been on trips where tent poles were forgotten, stakes, fresh food, fire starting items (matches, lighter, etc.), stove fuel, maps, sunglasses, gloves, warm hat...
The worst offense yet was New Years Eve 2010. I had decided to do a last minute quick overnighter with my dog once I found out my girlfriend's flight back into town had been cancelled. We got all packed up, made the drive to the trailhead and started down the trail just after sunset. We were only going about 3 miles in, so it wasn't going to be a big deal to be hiking in the dark. We got about 1.5 miles in and I realized I didn't have my camera. I wasn't sure, but I had a suspicion that I left the camera out on my back bumper of my truck. Grudingly, I decided to turn around and head back up the trail to get my camera. Upon returning to my truck, I discovered that not only had I left my camera on my back bumper but sitting next to it was my little stuff sack with my car keys, license, cash, ipod and cell phone inside. My sunglasses were sitting there too. Doh!
Luckily, once I'm on the trail, I've had pretty good luck about not leaving anything behind at camp. The only thing I can think of that I lost in this manner was my homemade tripod. I still can't figure out where I left it though...