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?
I know, I was hesitant at this too. ESPECIALLY when Bobby's wife died from being possessed by a demon. You'd think he had a charm. That being said, there is an explanation. Charms help, but they aren't foolproof. In season 8 Ms. Tran got a tattoo, but someone burned it off, enabling her to be possessed by Crowley. It's possible the same thing happened to Bobby- a demon got the jump on him, knocked him out, burned off the tattoo and/or removed the charms he had, and hopped into the vessel. When they'd be able to do this, I'm not sure, I can't remember the episode very well.
