Revealing mistake: In the scene where Rick returns from the lab with the macaroons, it is obvious that the worm man is only in half of his costume. He's wearing black pants and shoes and sitting on a stool.
Revealing mistake: Early in the movie, aboard the passenger jet, Stewie Gluck accidentally releases an emergency exit hatch and is sucked out of the plane (as is another passenger in a wheelchair). As the hatch stands wide open for a few seconds, with the wind howling outside, it's obvious that the "clouds" passing in the background are nothing but a large, billowing nylon sheet.
Answer: Honestly, I think it was just a bit of fun for Reeves. It's really not uncommon for prominent actors to do uncredited roles and cameos in films. I've seen it happen before many times. And given that the director/co-writer/co-producer is his "Bill and Ted" co-star Alex Winter, I have serious doubts they were worried about any "Bill and Ted" association. (If anything that would have helped them at the box office.) It was probably a case of Reeves doing it as a favor for a friend, and they decided to have fun with it and keep him uncredited to see if anyone would figure it out.
TedStixon