There is a myth about one man who supposedly was decapitated by an overhead track in 1979. Shortly after that, the ride was indeed closed for renovations and safety regulations improvements. Disney’s unofficial response was that this did in fact happen, but with a test dummy and not a real human being. The test dummy was left standing during the ride and was decapitated in a low area, so they decided to improve the ride to avoid a similar accident to actually happen.
Still today, there are areas in which taller people can reach out and touch the ceiling or the rails at Space Mountain, so what is keeping something like that from happening? (most of my info came from theorlandovacationstore.com)