Factual error: When Horton first crosses the bridge he breaks all of the planks. Then at the end when the angry mob goes to get him, they seem to have crossed the fully broken bridge to get to the other side.
Continuity mistake: In the scene where the Mayor is talking to Dr. LaRue, there is a rack of test tubes on the counter top. The position of the test tubes changes multiple times during this scene. This can be especially noticed after the Mayor spills the chemical and it eats away a big hole in the counter top, if you compare the position of the test tubes with the hole between different shots.
Audio problem: The chairman talks to the mayor, and says that "no one supports you." When he says "supports", his lips look nothing like it.
Continuity mistake: Horton looks back when he hears footsteps to show an empty field. Later, the Sour Kangaroo climbs onto a rock that was never there.
Continuity mistake: In the scene with Sally and JoJo standing in town square, at the beginning of the scene JoJo is as tall as his mother's first stripe. After Ned comes, JoJo is shorter, as tall as his mother's last stripe.
Continuity mistake: At breakfast when it's Jojo's turn with his dad, twelve seconds start on the clock, but when the mayor starts showing Jojo all the perks about being mayor, the clock goes back to twelve seconds left when it should only be at five seconds left.
Answer: There is a possibility that that bridge wasn't the only one to lead to Mount Nool, just the closest one.