The case
The Disney Channel launched in 1983 and Mickey was everywhere again: the ears, the gloves, the red shorts with the two white buttons. A surprising number of people also remember straps over his shoulders.
There are none. The buttons are the whole costume detail, and they're decorative, holding nothing up. The straps come from the mind's idea of what buttons like that are for.
Why our brains do this
Front buttons on shorts suggest suspenders, so memory adds the straps the buttons imply.
Sources: Wikipedia — Mickey Mouse · Wikipedia — False memory: the Mandela effect