When the flubber breaks out of Prof. Brainard's back pocket, it makes a hole. As he is falling out the window, there is no hole in his back pocket.
Towards the end of the movie, the pressure gauge on the container holding Flubber reads zero right before the Professor opens it, but the shot before that shows a positive reading.
When we first encounter Sara, she is wearing an engagement AND a wedding ring while talking about her wedding later in the day. The rings disappear in later scenes.
Inconsistencies throughout the movie of the flubber not bouncing all the time. Like when the basketball players all land on the court and only the one with the ball bounces up through the basketball rim.
When Brainard first discovered the Flubber and upon seeing the Flubber sneezing he commented that in the pot it was 77 "degrees" Kelvin.
Kelvin is an absolute unit, not a relative unit like Celsius or Fahrenheit. So the "degree" doesn't belong in that statement.
At the end of the movie, Brainard's neighbor and his son are flying in an airplane. The loud speaker says they are flying at 30,000 feet. The father and son see Professor Brainard and his wife outside the window at the same elevation. At 30,000 feet, the temperature is -150 degrees and the plane has to have cabin pressure because the oxygen level is too low for anyone to breathe above 15,000 feet.
At several points when Brainard is talking to Weebo, you can see the bluescreen reflected in his glasses that was used to composite in Weebo's video screen.
Midway through the film, the scene after Philip comes back home in his flying car and is talking to Weebo in his kitchen and telling her how much he loves Sara, Weebo is recording it. At some point while Philip is standing in that scene, we can see it's daylight out the window, but he just got back in from the outside being night time, further more to prove it should have been night time outside, he turns off the light to go to sleep and Weebo flies outside to go to Sara's home.
The Macintosh Plus used in Brainard's kitchen displays content in color. Every Macintosh Plus was built with a black-and-white display.
When Wesson and Smith are in Brainard's basement trying to steal the Flubber, Smith says something to Wesson and Wesson replies, "What?" But Wesson's lips aren't moving when he says this.
Air bag for catching stuntman is visible when Prof. Brainard falls out a window backwards.
Mountains are visible in the background when Professor Brainard is offering Flubber for sale to Ford, whose headquarters are located in the relatively flat Dearborn, Michigan.
In movie Prof. Philip is a chemistry teacher but in the starting of the movie when he enters in a wrong class (actually an art class) and starts his lecture, he describes a formula of physics.
At 1h21m12s, Wesson pulls a gun out of Brainard's pocket and promptly shoots him directly in the face with it, unaware at the time that it's a water-pistol.
The chess board in the kitchen is set up "sideways" - the white corner square should be on the right side.
How did Sara even know about Sylvia being created as a file name from Weebo when she wasn't even there.