Okay, so I've noticed this thing that's wiped me a few times in Uldaman.

One of my alt toons will sometimes target a mob that the main isn't targetting and fires a lightning bolt or whatever at it.
Something way far away from the currently tanked mobs. Not all of the toons do it, just one, and quite randomly.

This shouldn't be possible because all of my macros are like /cast [target=focustarget] whatever.
Maybe there is a lag delay in switching targets or something and it picks the closest one instead? I don't know. Wondering if anyone else has seen this.

Should I be doing '/assist focus' instead?