Not multiboxing, but the funniest thing I ever did was spamming Purge on a paladin near Icewing Bunker in AV. I had one of those add-ons that would show a message when I successfully purged a buff. So the paladin is taking a beating and decides to bubble and jump off of the ledge to get away from a group of us, and just as she leaps I see the add-on message:

PURGED: DIVINE SHIELD

(*Or whichever pally bubble it is that can be dispelled)

I got to watch as she grew smaller and smaller and then "paff!" And my target frame disappeared.