The OP did not just ask for 'missed' cameos but also any that are not well known. Granted, many people will be aware of this appearance, but some may not be.
Perhaps even more unfortunately: Steven's brother Cary became a .... very bad dude. Cary Stayner - Wikipedia How much must one family endure?...
George Raft in Casino Royale. He has one line and it's the best in the entire film! And that's no small feat considering my all time favorite actor is in this film.
A couple of famous people in crowd scenes: Phil Collins - A Hard Days Night Ruby Wax - Chariots of Fire
I've known about Spielberg in "Blues" for years - not sure if I recognized him in 1985 when I first saw the movie or if that came later - but I was today years old when I realized he was in "Gremlins"!
A friend of mine who was a big fan of the Lost In Space TV show didn't notice the several cameos by the cast members in the '98 Lost In Space movie.
Spies Like Us - Lots of great you-gotta-know-what-they-look-like to catch 'em - Martin Brest (Midnight Run) as an armed guard at the Ace Tomato Company, Ray Harryhausen (!), Sam Raimi, Joel Coen, Costa-Gavras.. Bob Hope, Terry Gilliam and B.B. King are more obvious.
One of the more bizarre cameos is Albert Finney's in the Coen Brothers classic, MILLER'S CROSSING. Waitaminit, isn't Albert Finney one of the stars of the ensemble cast, Leo the Irish mob boss? Yes, but he's also momentarily in the film in a cameo. He plays a woman in the washroom Tom invades early in the film to confront Verna. Apparently the washroom scene was shot after all of Leo's scenes had been completed, and Finney did it for fun, to spend another day on the set. Finney had been cast as Leo just before filming, due to the death of the actor originally cast. Now it's hard to imagine anyone else as Leo except Finney. It's one of his very best roles.
The guys that played inside the costumes of the original 'Teenage Mutant Ninja Turtles' movie from 1990, all made cameo appearances in the same film.
