Usually if we forget something it's some fresh food that was wrapped up and in the refrigerator (usually something that took time and effort to prepare). We generally take enough food that we don't miss it much, though.

Backpacking as a couple, at least there's some redundancy at times in the personal items (i.e., he forgets his headlamp, so he borrows mine), though stuff like dental floss where we share the dispenser, we designate who's packing and it gets packed then.

A watch. Every time. No pun intended!

but I had a magnesium bar anyway.

I prefer a king size Snickers bar. Tastes better.

Magnesium is an essential part of a healthy diet and of course multi use. That is : scape some on your morning porridge and light the fire with the rest.
Franco
Food in the fridge...
I did leave a bag some time ago. Now I put all of my food inside a large bag in the fridge the night before and leave the backpack open so that I can see the food is missing .
I only have my rain gear ,warm layer and my Nalgene Cantene (empty) on top of where I put the food. So quick to pack in the morning.
A couple of trips ago I forgot my main headlamp. I was charging the batteries and left it out.
But I had the E thingo, in my emergency bag..

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...

I used a check list, check my pack several times during the week grabbed put it in the car set my fuel bottle next to it in the back seat. Got to the trail head grabbed the pack and off I went without the fuel. Luckily I was meeting my brother in-law on the trail and he had esbit to boil water for my freezer bag meal

I have forgotten a few things here and there, like several people my sunglasses are the most common or sunscreen and for the same reason, no sun up that early so why would you even have glasses with you? haha.

Thankfully I really dont forget the important stuff. Probably worst thing Ive forgotten was gloves on a winter camp. Thankfully I had all the rest of my gear and borrowed an extra pair of fleece gloves someone happened to have with them and I was fine. There was little wind and wasnt all that cold so I stayed nice and warm :)

Never broken a hip belt buckle yet :)