Vampire movies: How come vamps always become stronger than their human form but the bones in their chests become jell-o???
Action/fighting in general: How come the hero is always fantastic in a fight until it comes to the big bad guy and then he gets his butt kicked by the lamest fighting techniques???
Bombs: Why do bombers always use timers instead of remote triggering devices? And why do the heroes always find out about the bomb seconds before its to go off?? Recent favorite scene countering this notion: The Chronicle TV series where they find the bomb, it seems to have seconds left on the timer and one character is all pumped to do something about it, when his friends show him that there's still two more hours that he didn't see and he was all shaky and coming off his adreneline and fear saying, "Oh good, let's call the bomb squad."
How come bad guys stay almost dead forever, rising up again at least one more time to fight, but the good guy cohorts always linger just long enough to say some cornpone. IF the good guy doesn't say anything and we're not sure he/she is dead, then he might help the protagonist by miraculously seeing someone sneaking up and shoot them dead from the ground, despite a near-fatal wound, a short barrel weapon, blood in the eyes...