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.
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: 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: 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)
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.