Tattoos

Following Sam's possession by Meg in 2.14 Born Under a Bad Sign, Bobby gives Sam and dean each a Charm Amulet which features a symbol which will stop demonic possession.

In 3.12 Jus in Bello, the boys reveal that they have had these symbols tattooed on their chests.

Sera Gamble on the tattoos origins

FFN: Those matching tattoos are pretty cool. How long has that been planned?

SG: I remember talking about it with Ben Edlund last season, while Cat was writing "Born Under A Bad Sign." We agreed that if we were Sam, the first thing we'd do is take that anti-possession amulet and head for the nearest tattoo parlor. He reminded me of it while we were working on “Jus In Bello,” because breaking out the amulets was an important story point in that episode. So I stuck it in there, and it made for a nice moment. It’s like, of course Sam and Dean have matching tats. How could it be any other way? By the way, I'll be convinced we're truly a cult hit when a fan gets the same tattoo.

Little did Sera know, that fans had been getting tattoos since early on in the Show's run.

Dean's tattoo is seen again in the opening scene of4.01 Lazarus Rising.

In 4.18 The Monster At The End Of This Book, this phenomenom is referred to when Sam and Dean reveal their tattoos in order to pass as avid fans. In response the publisher of the "Supernatural" books by Chuck Shirley, reveals she has the same tattoo - on her ass. While not named in the episode, the publisher's name was Sera Seige, named after writers Julie Siege and Sera Gamble - as mentioned in this interview by Sera Gamble.