I dislike when people do things, or have NPCs do things, that should not be done. Not int he, "oh, it's illegal" type of way - but the "How does that even happen?" 

For instance, in an early version of Allaria, an RP I run - the PCs were drafted into a war, as part of a plan to take down the big bads army that had took over most of the world and were making the final leap to complete dominance. So, both armies, good and bad, were standing on opposite sides of the field - the good barricading the city and the bad, preparing to attack.

The PCs are standing there talking about their plan. All of a sudden one of them launches an arrow... and ignites a ring of gas and whatnot, that not one... but TWO of the big bads armies had managed to just so happen to step in. Normally, this wouldn't be a problem, provided there was some prep for it. 

But, the battle hadn't started. There was no explantation of why the ring of gas was there. There was no explanation of how these two random guys were able to sneak past an entire army and get behind them, only to get caught by a, in terms of power as opposed to the bad guys, weakling. It was just an all around WTF moment. 

Only to be capped off by her jumping off the bridge, and into the river below, because their real mission was to get past the battle and head to the next city. That was her chosen route.