I've been to old Outland through the old deadmines by abusing the priest levitate bug.
I abused same bug to go above orgrimmar before cata.
I've been in the crypts before (love the place) using a bug where mounting and jumping at the same time would force you through walls so you could fly underground.
Same thing made it possible to get in next to the ah goblins in orgrimmar.
I've been in old Gilneas before cata (there were nothing. A big flat area....)
Tried to get into Hyjal before cata but you'd get kicked out :P
I were in uninstanced version of ZG pre cata re-release for the luls of it.
I'm sure i've been loads of other place but these are the things i cant remember right at this moment.
This is the kind of things that adds flavour to the game once you're able to say been there, done that to most things.
Still have yet to recieve a ban or a warning.