not on LJ, but someone once tried to convince me that he was right purely because he was part of a historically-persecuted group*.
I very calmly replied "I'm a Germano-Russo-Welsh Jew. I win."

he never again tried to tell people that we shouldn't argue with him (no matter what the subject matter was) just because of what group he was a part of.

* = part of a group trying to revive the Cathars, I think. it's been 7 or more years.