Leave Scenario/LFG group with no deserter.

[Sarick]

Firstly, I suggest getting addon like Quartz Casting Bar
Reason for that it is showing casting time more precise then standard, it shows cast in 0.1 and that is what we need.

So when you enter dungeon do this:
-Logout
-At 0.2-0.3 Leave group

Next time you log in your wow client should crash and you shouldn't have deserter.
Good luck.