In Supernatural season 5 episode 1 "Sympathy for the Devil", Bobby Singer becomes possessed by a demon.
From the wiki:
Bobby attacks and restrains Dean - he too is possessed!
Bobby is the one who gave the Winchesters the anti-possession charms in the first place. The brothers even get tattoos of the charm to prevent possession. Are we to understand that Bobby, a more knowledgeable and experienced hunter, had neither a charm nor a tattoo for anti-possession purposes?