When James Cole returns from his first trip to the past and is in his cell, he no longer has the bloody wound on his head that he received from his scuffle with the police in 1990. However, when he is being debriefed by the scientists in the next scene, the head wound is back.
hey man, this entire movie is based on the idea of time paradox and self correction of he timeline. Good point but I think all of that aside, maybe they cleaned it up and it started bleeding again?