Things I do/don't do that drive me crazy when I get on the water:

forget to mash down the barb on the fly - especially with small hooks, my forceps aren't the most efficient tool to use; the pliers I have on the tying bench are, as long as I use them. I've gotten pretty good about taking care of this before I even tie the fly but I there are some barbed flies in my boxes

forget to clean head cement out of the eye - again a product of being sloppy/careless/lazy at the vise; this usually comes back to bite me when I'm trying to thread on a #18 Iris Caddis in semi darkness. Until I got some nippers that had the little needle on the back end, I'd have to pull a used fly off my hat to clear out the hook eye

forget to clip off leader from the hook eye - instead of using those nice nippers I just mentioned, I usually just bite the fly off leaving that little tag of mono, which I then end up having to remove in semi-darkness or when the fish rises look like a hailstorm on the river

I'm not even going to go into keeping my fly boxes in order

Regards,
Scott