Stan Lee Cameo
This easter egg appears in all versions of the movie. Stan Lee's appearance is part of an on-going series of easter eggs where he appears in Marvel movies whose characters he likes.
Early in the movie, Senator Kelly is captured by Magneto and used as guinea pig in a machine used to mutate humans into mutants. After using his powers to escape from Magneto into the ocean, he later emerges from the ocean at a busy beach.
Stan Lee can be seen at a hot dog stand on the beach as a naked Senator Kelly walks past.