While car camping, I almost invariably forget -- the folding chair. Not sure why that item, just that I
always do!
Previously forgotten:
Tent poles! Ended up buying new tent because the old one was too heavy and was wearing out. I always keep the poles with the tent from now on.
Socks! Ok, I forgot these once. But they were in my car. It was just went I was resorting a few things at the trail head itself that I accidentally left them in the vehicle.
Toque! Just a couple of times but never in a really cold situation. They are cheap enough to replace in any case.
35mm Camera! But it's been a long time since I've done that since I've switched to a smaller digital camera.
Reading glasses! A few years ago when I was just getting used to needing these things I neglected to take them on a couple of trips.
I've never forgotten any of the "top 10" items on the list.