*Infidelity. Now if there was a one-time incident or a wandering eye, okay, I can deal with that as long as it's resolved in my pairing's favor. (E.g. I'm not cool with Hotch in a relationship while still married [i.e. not separated] from Hayley.) I know people cheat IRL but I'd like to hold my characters to better standards. =\
For me, that totally depends on the character. Given by how hard the divorce hit Hotch, I can't ever see him cheating on Haley. I firmly believe he loved her till the end. But for other characters, let's say Rossi, it wouldn't surprise me all that much. Infidelity is something that needs to be sold to me and more often than not, I just can't see it.