Flash jumping towards the camera screaming "YEAH!" was improvised by Sam J. Jones. Nobody could figure out how to end the movie.
All of the main cast members were signed for multiple films. The first movie didn't do as well as expected, so the sequels were never made.
George Lucas loved the old Universal Flash Gordon serials as a kid, and wanted to make a modern version based on the original comic strips. Federico Fellini was optioning the rights at the time, so Lucas wrote Star Wars (1977) instead.
In the finished film, George Harris' dialogue as Prince Thun of Ardentia was dubbed with a much deeper voice. However, Harris' actual, unretouched vocal track is featured on the Queen soundtrack album, indicating that the change must have been made fairly late in post-production. In the original comics Prince Thun was a lion man who was one of Flash's most loyal allies and wasn't in earlier drafts of the script and there was no attempt on Ming's life, instead Ming showed Flash, Dale and Zarkov how mindlessly loyal his guards were by having one commit suicide in front of them.
Max von Sydow's Ming costume weighed over 70 pounds. He could only stand in it for a few minutes at a time.