Other mistake: Just before the evil robots capture Bill and Ted's girlfriends, after they crash through the window, the girlfriends ask the robots: "Who are you?" and Bill goes "Well, I'm really Evil Ted!" and unzips himself and out pops Evil Ted, and the other robot goes, "And I'm really Evil Bill!" and out pops Evil Bill. When the robots are herding the girlfriends out just before knocking them out with their breath, you can see two sets of clothes in the bottom left corner if you watch closely.
Continuity mistake: In the scene where Death and Bill are playing Twister, Bill's left hand was on red and his right hand was on green. In the next shot when Death is putting his foot on blue, Bills hands are switched. In the same scene Bill's shoes are on the floor. After he wins they walk over to Death and his shoes are back on. He never had time between all the dialog to put them on.
Continuity mistake: When Bill, Ted and Death are at the gates of heaven, they are all looking up to the gatekeeper, talking to him. Bill and Ted are in the foreground, with Death behind them. However, when we see a shot of them from behind, Death is in front of Bill and Ted.
Continuity mistake: If you watch the concert scenes, the audiences are all different because it was partially taped at Six Flags Magic Mountain and Culver City Studios. Hollywood (From G.L.O.W. Ladies Wrestling circa 1980's) was sitting in the front row on the viewer's right at the S.F.M.M. taping, and she's not there at the C.C.S. taping.
Revealing mistake: In the scene where Bill and Ted arrive in Hell, they fall down backward when the Devil pulls a lever. If you look closely as they fall, you can see Ted (or his stunt double) holding a stick, perhaps to help break the fall.
Continuity mistake: After getting the fake phone call from Joanna (evil Bill using her voice), Bill and Ted are sitting on the couch in their apartment. There's a knock on the door and Bill puts his hat on sideways. But when he opens the door, his hat is on backwards without adjusting it while walking to the door.
Continuity mistake: When Missy opens the book in the seance, she is holding a separate piece of paper on the pages of the book. The close up from behind, over her hand, shows no piece of paper. When the camera is in front of her, she is holding the paper again.
Continuity mistake: When Bill, Ted and their girlfriends are sitting at a table talking, Ted says "You probably noticed we haven't given you our gifts yet" with Pepsi cans visible on the table. After a close up of Bill then Ted, the camera goes back to a shot of all four sitting at the table, and the Pepsi cans have changed positions. (00:12:50)