When Dusan is explaining his many business ventures, his cigar ashes disappear and reappear several times.
After making such a point that non living matter (hair, feces) cannot be downsized in the process, what about fingernails and toenails? They are made of the same dead substance (keratin) as hair, so technically the small people should come out of the process with full-sized, relatively giant nails.
While the Norwegian colony might not have needed a dome to protect them from mosquitoes and birds, other full-size animals would be able to get in. Also, full-size raindrops would be deadly to downsized people. More even if someone said they did not have insects because to close to the sea, we saw a dragonfly when they entered the village and also butterflies later (big ones !). And what about the waves? They live just by the water, any mild waves would shattered the pier. And what about snow that would cover them in minutes.
A person 5 inches tall would have a very high and squeaky voice - so high, in fact, that it would be on a different wavelength altogether and inaudible to the ears of a full-sized person, like an ultra-sonic dog whistle. (A dog might be able to hear them, but that's about all.) At the other end of the scale, a downsized person would hear a full-sized person's voice as a low growl, like a vinyl record played too slow. The downsized people all had "normal" voices - even in scenes showing them conversing with full-sized people.
Downsizing would only reduce material costs, not the cost of labor for manufacturing cars, appliances and building houses, etc.. What's more, all the equipment used in manufacturing would have to be created from scratch at the reduced scale, since only cellular organisms can be downsized. Said another way, even after all the infrastructure was in place, savings by downsizing would only amount to about 20 percent.
After Paul is downsized, the camera perspective switches to being from the point-of-view of a shrunk person, yet the clouds in the sky are relative in size to a full-sized person. At this size, a single cloud should cover the entire neighborhood, yet in numerous scenes many small clouds fill the sky.
Subjects are told they have to have all the hair shaved from their bodies, but the first successful subject--a lab mouse--was covered with fur when it was shrunk. It isn't that the hair hurts the patient (in the same way that left-in dental work does), it's that it would be insanely long because it wouldn't shrink with them. They would wake up seriously uncomfortable in a pile of hair, with every follicle stretched painfully.
At Paul's company farewell party, Leisureland on the cake is incorrectly spelled Liesure Land. This is clearly a joke (as well as some subtle foreshadowing).
Eyelashes are still visible on the just-downsized people. Per the rules of the story, they would have needed to be removed along with all other hair, or risk being a small person with huge eyelashes.
The movie goes into great lengths to show that everyone gets completely shaved head to toe. During the shot that shows the Leisureland employees removing the sheets over the men, one of the men still has visible pubic hair.
The clothing the little people wear seems to be made of the same materials as big people's clothing (for example, cotton). But even very fine cotton threads would be like ropes to the little people. If they somehow made the threads from downsized plants, they would need to create an entire supply and manufacturing chain to make cloth and clothing, and that would make everything as expensive as it was for big people.
The laws of physics change with scale; for example, because of the capillarity of liquids, champagne would not flow from a microscopic bottle.