Couldn't believe that no one else ever posted this slip-up!

During the (I think) second bedroom scene (the one with Brad), after "Janet" (Frank) gets into the bed, someone closes the bed curtain, but it sure isn't one of the people in the bed! You can tell that whoever pulled the curtain was below the platform.

Of course, this slip-up adds to the charm of Rocky Horror, and many theaters have incorporated it into their live audience call lines. But I laugh every time I see it!