At the airport, Kevin has his father's bag as he boards the plane for NYC. Then it is by his own bag showing up at the baggage claim, in Florida, that Kevin's family realizes he's not with them. So he wouldn't have had the inflatable clown with him to use in the Plaza as a ruse in the shower. It wasn't in his father's carry-on bag, so it would've been in Kevin's bag that went to Florida.
When Kevin is recording the Plaza Hotel commercial with his TalkBoy and it says the toll free number, Peter walks in and asks about the battery for the camcorder, overlapping it; yet when Kevin plays it back at the hotel, Peter's voice is not included on it.
When Kevin is on the bed in the hotel room, we see the butler scooping ice cream into the banana split boat, then the butler asks Kevin, "How many scoops would you like, sir?" and the banana split boat is empty and the butler starts to fill it up.
The recording of Uncle Frank singing in the shower is much longer when Kevin plays it back in the Plaza Hotel. We hear what was sung before and after Kevin started and stopped the recording.
When Kevin meets Marv and Harry outside the toy store, he drops his map and magnifying glass on the pavement. When he later runs away, these items are not there.
Factual errors
When Kevin soaks the rope in kerosene, he later lights it on fire to deter Harry and Marv. Given the evaporative nature of kerosene, the volatility would be so low, the rope would barely light, if at all.
When Kevin goes swimming at the Plaza, it is a different hotel. The Plaza doesn't have a pool.
When Harry uses a toilet bowl (unknowingly filled with kerosene) to extinguish his burning hair, he positions himself vertical in order to reach the liquid; this would have lit his clothes on fire, given the vertical flame.
The police officer who questions Peter and Kate at the Miami Airport about Kevin being missing should had arrested them for child negligence, especially after they admitted a similar incident involving Kevin happened the previous Christmas. A qualified police officer wouldn't ignore the fact that parents are treating the incident as a laughing matter. This would give the police the assumption the minor has neglectful parents.
When Kevin takes a picture of the criminals robbing the toy store, his pictures were clear at the end of the film. That type of camera has an instant flash, and would have made a blinding reflection off the toy shop window, revealing nothing in the finished picture other than a flash of light with a black background - not a clear picture of the crooks.