Plot hole: During the episode "Superstition", Onizuka thinks he has cancer because the magnetic pain-pads he was wearing created strange blobs in the X-Ray he has taken. However, given the fact that the X-Ray was so strange, doctors would have given him a complete physical, and noticed the pads much sooner. Here, they literally just assume he has tumors, and fail to ever take notice of the pads. Simply impossible, given the gravity of the situation. (Obviously, this is to pay off at the very end of the episode.)
Other mistake: During the first dozen or so episodes, during the opening credits, Onizuka is shown spray-painting a picture of a woman on a wall, with the words "Love" and "Piece" written next to her. Obviously, "piece" is a mistranslation, and the wrong spelling. The correct text should read "Love" and "PEACE."