Worse than forgetting something, which if you are lucky you only do once --

Continuing to drag around "anchors" just because you might need them, or because you have always carried them.

A friend of mine can not, will not, won't go camping without his cast iron frying pan. I think it weighs as much as 1/3 of my gear all by its lonesome, is awkward to pack, stays hot forever, and is a pain to clean. Yet time after time, he has to have this pan.

Another guy was carrying around this giant 5 D cell maglite. We were kidding him that if the airport beacon went out they could call him. That and if a bear ever attacked he could whack it senseless with his flashlight.

Oh, the one thing that I never ever forget - TP. One of the first things in, seldom taken out, often replaced. Leaves just don't cut it.