1. If you commited a crime - Theft, Vandalism, Murder you leave scents that can be tracked and the tracker can force summon you even if you're offline.
You can be beaten, your stuff taken and be killed.
Only way to prevent that is not leaving scents...
2. On very few occasions i heard ppl where attacked just in the moment they're logging out (i never experienced it myself) - there's a minimal time gap between " pushing the log out button" and really being logged out...
You didn't state if 1. or 2.
1. is part of the game - 2. is just kind of exploit (or cleverness)