Continuity mistake: At the beginning and end of the film, there is a narration who the subtitles label as Sitka, but at the very beginning, we see that the narrator is actually Denahi.
Continuity mistake: After Koda frees Kenai from the rope trap, the rope disappears twice from around his (Kenai's) leg: first when he is walking towards his brother Denahi (then the rope reappears around his leg as he runs away), and disappears again when he meets Koda again inside the glacier (only to reappear again when he snags it around a rock/ice chunk and trips).
Continuity mistake: In the scene where the bears are riding the mammoth, Koda is touching Kenai, but when the camera angle changes, they are at least a foot away from each other.
Continuity mistake: During "On My Way," a shot in the song shows Kenai and Koda in the rain. Kenai's fur is soaking wet, but Koda's fur seems nice and dry, which is impossible if they're walking in rain.
Other mistake: At the end of the film, as Kenai walks up to Koda after he's been transformed back into a bear, Koda leaps on him in joy. If you look closely at Kenai's hind legs, there are a few frames of animation missing. His legs go from a standing to a sitting position instantly. It's barely noticeable (no pun intended).
Factual error: During the opening scene when Kenai is running away from a herd of stampeding caribou, several of the caribou do not have antlers, even though both male and female caribou sport antlers. One of these "antlerless" caribou is even seen running near the front of the herd and is easy to see.
Answer: The bear injures Sitka right before he sacrifices himself. We don't see the injury itself due to the film wanting to keep its G rating, but we do see the bear attack and most likely maul Sitka, heavily implying his injuries are fatal. This is why he's hunched over and breathing heavily when we next see him as the bear is moving towards Kenai and Denahi.