A couple of my friends once set their names to Calvin and Hobbes, and then played heavy and medic respectively on TF2.
Another one was when playing L4D (as Zoey), one friend set his name to: 'Damnit Jim, I'm a doctor not a' and then ran out of space. Another friend (who was playing Louis, and was thus next to him on the HUD display at the bottom), promplty set his name to 'prostitute'.
The best thing about this was that the continuation of the two names was perfect: there was literally no gap between the names. I'm a bit annoyed I didn't get a screenshot really...
edit: another of the more surreal ones was (again on l4d) setting our names to the name of the character we were playing, and then changing our names to names of the infected when the teams switched, and then changing our names back to the survivors' names, but mixed up (ie, I was Zoey whilst playing Bill)...