I would say tilting is at the top of my list of player problems but there's one other negative emotional response that can be be more difficult to deal with. Tilting from a bad beat is something we all have experienced and many of us have at least learned to recognize it and try to deal with it. There's another emotional response that is a lot more insidious because it builds slowly and is hard to recognize.
You're following advice from poker coaches and watching the 'weaker' players who fold a lot and, in particular, call pre-flop and then fold to a c-bet. You've seen their bad play and now you're feeling a little superior. You're going to take chips from these guys. You pick up a hand, say AJu and you make a 2bb bet to see if one of the losers calls. Sure enough one does. The flop is T74 rainbow so you put out a tidy little c-bet and the loser calls. Hmmmm your superior play didn't quite go as intended. But you know how much better you are so you just have to bet bigger to get that mouse to fold. The turn brings another 7 and you bet big. And then the loser shoves....oops. Great. He probably has a set or two pair. Most players will fold here and look unhappily on their diminished stack. Some will turn that superior attitude into a stubborn, losing, call.
The mistakes were cause by emotion, just like tilting, but a lower level that is harder to notice. And the damage can be just as bad as from tilting.