Continuity mistake: The whites of Simba's eyes change frequently through the movie, from yellow to white. They're yellow up until the scene where Pumbaa, Timon, and Simba are stargazing; after that they change back and forth frequently. During the Mufasa-in-the-sky scene, when Simba asks, "How can I go back? I'm not who I used to be," they change from yellow to white and back within three frames.
Continuity mistake: At the beginning when Rafiki puts the paint on the picture of Simba (on the tree trunk) the paint goes on a different color than it is. The film makers admit this mistake on the commentary.
Continuity mistake: During the stampede in the gorge, when Simba jumps up on the dead branch, he leaves very distinct claw scratches. Then in the following scenes they're gone, although other marks and details on the branch remain.
Factual error: When Simba and Nala meet as adults, they rub heads and begin to purr. Lions can purr, but they only purr when they exhale, and Simba and Nala were purring when inhaling and exhaling, like house cats.
Continuity mistake: When Timon rides Pumbaa through the vultures that are crowding around Simba, they run right over where Simba should be, but he's not there. 2 seconds later, after all the vultures are gone, Simba is back.
Deliberate mistake: Rafiki is supposed to be a mandrill, but mandrill's do not have tails. In the recently released Extended-Version DVD, there is a commentary by an animator who specifically admits that he knew that mandrills don't have tails (at least not like the length that Rafiki sports throughout the movie), but he put one on him anyway to make him more monkey-like.
Continuity mistake: When Simba and Nala fall through the dead elephant, there are some small rocks near them. But after the hyenas say, "Here kitty kitty kitty," Simba makes a small roar and the rocks are gone.
Factual error: When Mufasa is showing Simba the kingdom from Pride Rock they are facing the sun, yet Pride Rock's shadow is visible below them. Also in the same position, you see that they have shadows behind them. Then a few seconds later when Simba sees the shadow area the shadows are in front of them while you can see part of the sun to their left side. They would be facing south when the sun is east. How are their shadows in front of them?
Continuity mistake: When Simba and Nala are in the elephant graveyard, Simba goes to see if "it's brains are still in there" and he's past the base of the trunk. But when Zazu pops up in front of him, he hasn't passed the base of the tusk yet.
Continuity mistake: When Simba walks away from the jungle and Timon says "He looks blue," Simba is at a fair distance from the shadow of the tree. But a few shots later, Pumbaa comes to Simba and asks, "Kid, what's eating ya?" but Simba is now closer to the shadow of the tree.
Continuity mistake: When Rafiki is showing Simba where his father is, Simba is following Rafiki through the trees and they arrive at a river/lake. Rafiki says 'shh' and pulls the grass apart. In the shot facing him and Simba, Rafiki is only holding the grass on the left back, but the grass on the right is bent back the exact same way, without anyone holding it back.
Audio problem: In the scene where Sarabi gives Simba and Nala permission to go to the waterhole, Simba yells "Yeah!" and you can distinctly hear that Nala says "all right!" But it's animated so both Cubs appear to be exclaiming "Yeah!"