When you die and resurrect, you will be invisible for about 10 sec. Now if you logout during this time, then log back in, any mob who were close to you, cannot see you unless you attack them. Useful for interacting with heavily guarded objects, like chests etc. that would be hard to solo otherwise.
EDIT:
I just found out even better method, you don't need to die, just press the run button, then click logout, it shows the timer window where you can choose immediate logout, keep running until the mobs aggro, then click "immediate", and when you log back in you're out of combat and the mobs won't notice you even if you're 40 levels below.
PS. logging out also resets the resurrect wait timer to 30sec.