Biggest photo mistake: Seeing something I would like to shoot and not having my camera, or worse, having the camera and telling myself I'll come back and get the shot later. It never, ever works out either because I can't get back or by the time I do the moment has passed and the scene has changed.
Probably contender for the top spot: Calculating the per-shot cost of my current 6x7 medium format camera. Film can run as much as £20 a roll, the camera can hold 10 shots, and processing is another £12. On a camera that currently goes for around £1,500 it's pretty hard to justify.