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.
Trivia: In the scene where the Who's finally discover that Horton is real, the Mayor begins introducing random townspeople to Horton. When he introduces Bert, watch closely: Bert is drinking from a mug that has the Blue Sky Productions logo on it, one of the companies that produced Horton Hears a Who.