Linden Lab claims that with the newer server software, ghost avatars are a thing of the past. I'm not so sure...some bugs seem to creep back in from time to time. One of the most frustrating ones is ghosting.
When this occurs, usually after a crash following a teleport attempt, you can no longer log in to SL. If you look at your online status, the system thinks you are already on line. And in fact, if someone goes to your location, there your avatar will be, silent and unresponsive.
The most complete information I've ever seen on how to fix this problem can be found here:
Kudos to Nalates for compiling this information!