Plot hole: In the Ice Tunnel scene, Manny, Sid, and Diego come to a stop on the vertical block of ice. It breaks, and they slide through a 'field' of icicles sticking out of the ground. Their 'sled' is shaved down to nothing, indicating that the icicles are sharp. After going through the same tunnel as the baby, how did the baby make it through that field?
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: 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.
Continuity mistake: When the Sabres are chasing Sid on the skis (made out of two slabs of rock), Sid loses one of the skis, one of his feet is then flapping about and the other one is on the back end of the ski (which then becomes a snowboard). In the next shot(no time between them) Sid has his foot halfway up the ski.
Other mistake: After Scrat's landing you can see three pair of feet who are stomping on him. At least two of the three pairs belong to tetrapod vertebrates (as can be seen when the camera pans up to behind the herd), so where is the second pair of their respective owner's feet?