When the Hawks are chasing Professor Brainard and Betsy, the two policemen are shown approaching the area in their squad car, a 1956 Ford. However, when they arrive on the scene, they are driving a 1960 Ford.
When the professor is explaining to Charlie how flubber will power the Model T, it is obvious that the engine and transmission are missing from the car. He then takes the car out for a test drive/flight. The bottom view of the car clearly shows the engine and transmission in the Model T.
When Alonzo Hawk is talking to Ned and Betsy about 'merging together', Betsy is putting on his flubberized shoes. Right before Alonzo gets up you clearly see that the soles of the shoes do not have Flubber applied, although he jumps as if he did. He never changes shoes.
While Ned and Betsy were flying over Washington, the Capitol dome changed from white to dark and then back to white. That dark color was primer that preceded a fresh coat of white paint, and it was that way for far longer than the duration of that scene.
When taking flight, while on the ground the wheels will turn due to being on the ground, but once airborne there is no friction with the ground to keep any of the wheels turning, yet all four wheels keep turning.
Amazon incorrectly states on the Contributions section for the widescreen DVD that Kurt Russell contributed to this film. His first Disney film was Follow Me, Boys!, released in 1966, not this film.
Professor Brainerd and Charlie WERE belted in when the Professor took the Model T on its maiden flight, although it is very hard to catch. Thus they do not fall to their deaths from the aerial somersault.
Apart from Tommy Kirk, all of the college students are too old.
When Ned is extricating himself from the harp, the bottom of the heel of his other shoe is clearly visible. It has no Flubber on it.