(at around 33 mins) When we see Wayne and the rest of the adults falling down the laundry chute with the shark cruiser, the bands they use to hold themselves in have disappeared completely.
(at around 1h 3 mins) Trey turns on the stereo twice.
(at around 32 mins) When the parents are in the toy car riding on the track there is a scene where a bulldozer is about to wipe them out but in several shots the car has yet to cross in front of the bulldozer yet between those scenes the car has already passed the dozer.
(at around 15 mins) When Wayne and Gordon move the Tiki Man into the attic, Wayne closes the attic door. However, after he turns the shrinking machine on, there is a shot of the Tiki Man, and the door is clearly open.
(at around 1h) When Mitch discovers Diane and Patty on the table, there is a shot where he bends down and says "Mom?" In this shot, Patty is on the left, and Diane is on the right. It cuts to both of them nodding, and they have switched sides. Then it cuts to Mitch fainting, and they are back in their original positions.
(at around 9 mins) Wayne suggests drinking milk to disperse the glowing liquid throughout the body by using the "calcium channel". Calcium channels are found in muscles and bones and help regulate muscle and nerve function. It's not a transport function.
Severe hypokalemia (low blood potassium) is a little more dangerous than the film makes out, and certainly can't be fixed with a few mouthfuls of banana (and not that quickly). More realistically, you'd need a couple of hours on an IV drip, and preferably have a heart monitor watching your heart rate, too.
(at around 19 mins) When the Szalinski brothers are fighting after being shrunk, they turn around to see Diane walk in the attic. As they turn around, you can see their shirts cut off from the green screen chroma key effect at the right of the screen.
(at around 10 mins) Gordon shows his stomach and has no belly button.
(at around 1h 1 min) After Mitch has fainted and Adam and Jenny are feeding him a banana, Jenny is speaking to Mitch but her lips aren't moving correctly.
(at around 33 mins) When Adam and Mitch are playing with their chili volcano and it is getting ready to "erupt," a crew member's hand can be seen pushing the balloon back down into the volcano.
(at around 56 mins) When Gordon is climbing the stereo, in one of the shots, at the top, left-hand corner of the screen, you can see the top of the stereo "wall".
At around the 25 minute mark, Patty's car rolls out of the driveway and crosses the street into the driveway on the other side, out of sight. The children immediately come into view a second after this happens, seemingly oblivious that the car was ever there. If they were rollerblading down the street in that moment, they would have seen the car roll backwards across the street and know the mothers had returned home.
Wayne and Gordon fall in the dip just as the girls are using it. Given how close the girls are the dip and Wayne & Gordon swimming out of harm's way, it is a major surprise none of the girls saw two tiny things swimming in their dip.