Ooooh thanks! I didn't notice that at all. Maybe he accidentally slammed the car door on it? That happened to my son once and the whole finger swelled up like anything. Or maybe, like you said, the treehouse!!

But that was the middle finger, not the ring finger. Though he probably injured his ring finger as well. But I wonder why Kripke didn't say why when someone asked him why Dean wasn't wearing his ring. I think he said it's not important to the storyline or something, and he didn't elaborate further.