Revealing mistake: In the park after the bad guys have been defeated and everyone starts returning to the museum, watch the T-Rex closely. In the overhead shot, he seemingly "floats" right through the branches of the trees which he normally would be moving. (01:34:50)
Revealing mistake: When the T-rex launches the table away, a couple of papers on it don't move a single inch, revealing they're glued to the table.
Revealing mistake: Towards the beginning of the film when Ben Stiller looks at the photo at him and his son, you can see that the photo is a fake. Ben and his son cast a shadow on the shadow behind them.
Revealing mistake: Obvious fake snow when Cecil runs away and the cowboy shows up in the remote controlled car.
Answer: Columbus is speaking Italian. The first night, when Larry asks him about the lions, he says "Go find the lion somewhere else" and then "tourist!" The second night, when they're chased by the animals, he says "Young man, do your job well, take care of the beasts" and when he sees that the window's open, "Kid, what do you say, shall we close the windows?" The last night, when Columbus is guiding the Neanderthals, he says "Fellow Neanderthals, with me!" and to the night guard "It's over!"