According to this unconfirmed and highly dubious legend, in 1936, shortly before Walt Disney's 35th birthday, his brother Roy encouraged Disney employees to hold a surprise birthday party for him. They complied, and a surprise party was organised. Two animators (whose names, to this day, have not been made public) decided that it would be funny if they were to animate a short Mickey and Minnie Mouse "Sex Tape" of sorts for the surprise party. When the short was later shown at said party, Disney responded by feigning laughter, before complimenting the quality of the animation and inquiring as to who had created it. Once the two animators came forward, he immediately fired them, then left the building. It has been claimed that he ordered the destruction of all known copies of the animation shortly thereafter.
It is extremely likely that the short never existed. The sole original source of this story is Marc Eliot's 1994 Disney "biography" Walt Disney: Hollywood's Dark Prince, a book which has caused a significant amount of controversy, and whose Walt Disney "facts" have been heavily disputed by a range of other authors and past colleagues of Disney himself.