When Dean and Sam are talking in the motel, and Sam's telling Dean what happened with Ruby while Dean was in hell, Sam has changed t-shirts and washed away the blood from his earlier injuries. When he was stitching up the cut in his arm, his t-shirt is light gray. Time passes while he's talking to Dean, and he has visibly changed shirts to a light blue t-shirt, and has washed the blood off his hand/arm as well. Even the lighting in the room is different, indicating time has passed. There is no continuity error, as mentioned elsewhere; time has passed, instead.
When Dean and Sam return to their Motel room after meeting Allistair for the first time, Sam is seen closing a major wound on his left arm with sutures. The whole thing is quite bloody and stains his shirt. He then sits down with Dean and explains how he started hanging out with Ruby. Everytime the story goes back to Sam and Dean talking in the motel room, Sam's left arm is perfectly clean and no wound or sutured wound is visible.
When Ruby is teaching Sam to exorcise the demon, and the subsequent scene, Ruby walks into, or across the Demon Trap with no consequences.
When Sam and Dean are talking to the nurse at the hospital, they discuss the Apocalypse. They name the book of the Bible, "Revelations", it's actually "Revelation".