The one I remember most was finding out I forgot the breech block for my Big Rifle at a Cheyenne Wells 50 cal shoot. I had other guns to shoot and I mainly helped at the shoot anyway so it didn't ruin it. I was kinda bummed about it though. I've forgotten that thing twice.
I know I've forgotten things other times but that's the only time that jumps to my mind. That's why I always bring more than one gun when I go shooting. It makes it less likely that I won't be able to do any shooting.
Have fun