) It happens a lot of times: prepare everything, set all the settings right, all flashes and the camera, speak to the model, bla bla bla, prepare to take the picture and "bam", no focus and no view: lens cap not removed. Everyday stupidity;)
p.s.: A lot worse when it happens in a crowd;))
p.p.s.: Or: walking around with an off-camera flash in hand, taking snapshots for a few minutes, than realizing that the flash isn't turned on;) Woooooohoooo!