Everything in Its Right Place - S4-E11
Character mistake: Lucas is seen placing flowers on Keith's tombstone, which states he died on January 23, 2006. In the next shot Lucas places a single flower on the grave of Jimmy Edwards, who died the same day and in the same scene as Keith, yet his tombstone states that he died on March 1, 2006.
Everything in Its Right Place - S4-E11
Character mistake: Brooke and Rachel are in the clean teen meeting and they are asked if they want to join. After both saying yes, the clean teen president hands them their shirts but calls Rachel Brooke, and Brooke Rachel. You can see their hair colors and they didn't change spots when she went to get the shirts.
Character mistake: During the state game, Mouth starts to announce information about the game. You can see him in the background, however, his mouth is closed and he isn't speaking until it changes to the next clip. (00:28:00)