Continuity mistake: In the first 'Ice Age' Diego isn't afraid of water (in the beginning he chases the woman and the baby through the river before she jumps off the waterfall and never shows any fear) but in this one he suddenly is. It's not like that he is just afraid of swimming because in the second movie he shows fear anytime when he touches water.
Plot hole: At the end of the movie, when the water drains away, there would be no way for a) the amount of water to drain away that did and b) for the mammoths that came through to be completely dry. (01:16:20)
Continuity mistake: In the scene where Scrat gets attacked/attacks the piranhas, the acorn gets swallowed by a piranha on the right, after all the fighting, Scrat jumps on a piranha on the lower left, which spits out the acorn. (00:35:20)
Continuity mistake: Near the very end of the movie, we see that Scrat's acrorn holes as he goes up the iceberg wall are in a fairly straight line. However, when the ice begins cracking, the cracks follow the line of holes which are now no longer in a straight line but very jagged. (01:13:05)
Continuity mistake: Right at the beginning of the movie, when Scrat is looking for his nut, there are no teeth marks where he stuck his teeth in to climb. (00:01:35)
Other mistake: During Scrat's encounter with the rotating ice slab, he jumps three times in the middle of the slab and falls through. He should have jumped on either end of the slab. Also, he fell through the ice standing up, and in the next shot he is upside down, pushing up on the ice slab.
Other mistake: Watching the green fish monster closing up to Sid and Diego, viewing the monster from underneath it, it would have taken approximately only one second for it to have reached them in the rate it was displayed traveling at (considering as of the end of that shot). But it only reaches them about eleven seconds later. (00:31:05)
Continuity mistake: As Sid says "Yeah. She's tons of fun and you're no fun at all", Diego is on the right side of Sid (from Sid's point of view), standing relatively close beside him. Immediately in the next shot, when Sid finishes off by saying: "She completes you", Diego no longer can be seen beside him. (00:34:45)
Continuity mistake: At the beginning of the film you'll notice that when the blue creature from the ice chunk dives back into the water he has a tail fin. When the creatures attacks Manny and his gang and Manny shoves the creature off it dives back into the water again without a tail fin. Then at the end when Manny is attacked for the last time the blue creature has its tail fin again.