No mistake is as timeless or universal as raising the camera to your eye, seeing nothing but black, then taking the lens cap off and trying again.
This is probably the #1 reason why I upgraded to SLRs from my (pre-Live View) P&S bodies, with their separate viewfinders: too many photos of nothing, because I left the lens cap on. Have also unintentionally left the self-timer on, on more than one occasion.
Went to an outdoor arena a couple of months ago, all set to capture a few choice images of one of my friend's performances (community theater). Camera, two lenses, monopod, spare batteries ... no memory cards! None in the body, none in the bag, none in my pockets (and none in the car, either!). The only thing that garnered more attention from the members of the audience around me than my setting up the gear, was my breaking it back down and putting it all away before the performance even started!