Factual error: How is it that when Sly comes out of the freezing, icy water, his hair and clothes are bone dry? I don't know too much about hypothermia, but I would imagine that his fairly thin sweater isn't wind proof. I'm sure by the time the movie ended, he would be suffering from more symptoms of hypothermia other than chattering teeth.
Factual error: John Candy is at the top of the Calgary Tower having lunch. In the background you see a building called Bankers Hall which is slightly higher than the Calgary tower, that's fine except this was the 1988 Olympics, that building wasn't completed until late 1990.
Factual error: In the early scene where Dennis Nedry meets a man to plan the embryo theft, the subtitle says that they are in San José, Costa Rica. However, the sleepy coastal town in the movie within sight of the ocean bears no resemblance to the real San José (a cosmopolitan capital of almost 1 million people, situated 50 miles from either coast). Even the book makes this error. (00:13:10 - 00:14:45)
Factual error: Not long into the film, D'Artagnan jumps in some hay. There's a flag of the province of Québec (Canada) visible in the background. This film is set in France at a time this flag didn't even exist.
Factual error: Most of the time the boys have no visible breaths, even though they're in a freezing climate.
Factual error: During the movie, characters may use lingo that would not be understood easily in the time period of their counterparts. April for instance uses the word "cool" in the modern sense, and nobody questions that, but you could assume that they don't understand every word and infer nonetheless the meaning from the context. Walker however when he's about to fire his cannon uses an idiom "Try this on for size" that was coined in early XX century. (01:19:40)