"Teleporting" all over the map isn't just for hackers these days -- an issue with the way Blizzard synchronizes data between the servers and the clients allows anyone to mysteriously jump from place to place without the use of any third party software. I'm sure we've all been in groups where we see someone start to run forward and then continue running forward, right through a pack of mobs and into another and another... there's a moment of panic before you realize that, oh, they've just gotten disconnected, and none of those mobs noticed him at all.
What's happening is that Blizzard's servers know the person is disconnected (they're no longer synchronized with the server) and hasn't actually moved -- but on everyone else's screens, the character continues to do exactly what it had been doing previously. The purpose here is to keep the game running as smoothly as possible in a world of imperfect network connections. When someone experiences a lag spike and is de-synched from the server, Blizzard made the decision that they wanted game performance to look smooth to the rest of the players in the area -- so instead of freezing the de-synched player in place, the de-synched player continues doing just what they were doing before (at least on other player's screens). When the player re-synchs with the servers (he's no longer lagging, or he reconnects after a disconnect), Blizzard takes the client's data as the most accurate interpretation of events -- meaning that even while lagged, you can continue to play and issue commands as usual. (In this sort of system, you may not even notice lag unless it's quite extreme.) When you re-synch with the servers, Blizzard accepts what you were doing and synchronizes the game world to that. And for the most part this is entirely invisible to players, except for those few times when a lag spike turns into a longer disconnect.