I have noticed this more frequently lately as well, even more so since 2.4.3. I have seen it both single and multi boxing. A couple RL friends have complained about seeing this as well. What I will see is; fighting a mob and the mob will stay still and not appear to lose health. It will die and you will not be able to loot it as you are "not in range". Other things that go along with this is "not facing the right direction" when it looks like you are, pets attacking far from mobs but still doing and taking damage and dead mobs sliding across the ground (rare).

This use to happen ALL the time in the first year or so of WoW. I once had an invisible mob chase me all the way across stranglethorn. Unable to do any damage to it and unable to drop combat. I finally died and had to relog, came back in somewhere totally different then where I had been.

You typically will not see any other types of symptoms. Ping will look good, in game and out, traceroute wil look normal. Other internet use will be fine.

It is a client/server desync. The only way I know to fix it is to relog. A number of things could cause it, including all of the things listed above as well as addons and most likely server side problems.