A revision ghost is a revision that a branch knows about and does not have. Revision ghosts can occur in unusual situations such as the migration of branches from foreign revision control systems into Bazaar-NG format.
Revision ghosts can be scary the first time that a user bumps into them. Thankfully though, revision ghosts are just like real ghosts (such as they are) in that their discorporeal existance typically causes little, if any, harm.