Continuity mistake: In the scene where the Professor and Sara are fighting the bad guys, Will Wheaton's character throws an ashtray at the Professor. Before the ashtray gets to him he sprays his rear end with the liquid flubber from the squirt gun. When he does this he sprays his jacket which at that time is covering his rear. When he jumps and hits the ashtray the ashtray does not hit his jacket but hits his pants. According to the movie flubber in any form needs to come into contact with the object to bounce back.
Continuity mistake: When the scientist throws the balls out the window, they go diagonally into the sky but then they come back down and hit the men who are standing in the place where they took off from.