Other mistake: When the holographic human version of Weebo leans down towards Philip while he is asleep, the reflection of the woman in Philip's glasses is not reversed; note the hair and the way she tilts her head. (00:42:30)
Other mistake: In the scene where the Professor and Sara are fighting the bad guys, Will Wheaton 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 his pants. According to the movie, flubber in any form needs to come into contact with the object to bounce back.
Chosen answer: It was rice and he just simply dumped them on the floor because he knew he wouldn't be needing it after the wedding to throw at the bride and groom because the Professor didn't show up.