I searched for this and haven't found it documented, so here is a bug that has happened to me a number of times, though I haven't connected it to any particular action or sequence:
At some point in a game, I notice that the AI of the mobs in the game change so that they don't recognize my Merc, and they all attack my main character. The monsters will walk right by my merc and attempt to attack me no matter what the relative position of both of us is, and this isn't changed by porting out or switching acts, though creating a new game will remove it (I haven't tried exiting and rejoining). This can be very annoying when playing a sorceress or a necromancer. This has happened to both my sp characters and my battle-net characters, with no mods installed at all.
Has anyone else experienced this bug?