Obviously, this thread will contain some kind of spoilers - but most films mentioned here will most likely be crap anyway, so who cares, right?
I'm not talking about plot holes now (though you can mention them too if you like), but things that happen that are just so stupid, that you can't forget them.
For example: King Arthur (the Clive Owen one):
Yes, an incredibly bland and cliched flick, but if the Celts couldn't get past the wall, how was it that suddenly, right at the end, they joined the fight from south of the wall? Nobody let them pass, but they somehow found a way through via the woods to attack the joint enemy to turn the tide of battle... Sigh.
I could string together a whole list now (the entire horror genre is ful of them!), but I'm hoping that some of you will chime in...