Continuity mistake: When the squirrel gets stuck between the 2 glaciers he gets stuck at the bottom of them, but then it shows him getting fired out of the top of them.
Continuity mistake: The scene where Scrat is trampled you see the feet of a camel-like animal and the feet of a mammoth. When the camera pans up, the mammoth is nowhere to be seen.
Other mistake: The scene where Manny builds his shelter. When Manny is bothered by what Sid is saying he turns around. The shelter that Manny has build is not large enough for him to turn around in it unless he walks out and turns around, which he does not do.
Continuity mistake: Near the beginning of the movie, when Sid decides to return the baby, he starts trying to climb up the cliff. The baby falls out of the little sleeping bag. After that, we never see it until it's used in the plan to escape the saber-toothed tigers. Animals don't have pockets, and I don't remember seeing anyone carrying it around, so where was the sleeping bag in between these two times?
Continuity mistake: Scrat lands on the main road and is not stepped on for 16 seconds. During that time, not one animal passes by. Yet once he is stepped on, the animals pass by at 2 to 4 seconds each, and when the camera pulls back, you can see dozens of animals. Scrat should have been hit as soon as he landed.
Continuity mistake: Just after they have finished meeting Diego, Manny says to Sid, "Okay I'll help you return him, but promise me you'll leave me alone after that" and Sid says "Ok ok deal", but as he says "deal" the shot changes and they are further apart then they were before.
Continuity mistake: At the part where Sid, Manny and Diego are at the "shortcut" Sid says "I choose life" and walks out of the shadow of the glacier thingies. In the next shot, the shadow is completely gone.
Continuity mistake: When Sid is climbing up the wall, he is having trouble at a metre above the ground, Manny starts to say something. Mid-sentence the shot goes back to Sid and he is twenty metres up.
Continuity mistake: When Sid wakes up and finds out that his family has abandoned him again, a animal in the process of migrating is walking by him leaving no traces of anything. But when Sid takes his next step, he walks into some feces he claims belongs to the animal that just walked by him. But the animal did nothing at all, the feces wasn't even there to begin with.