Deliberate mistake: At graduation, after Troy is handed the diploma, in the overhead shot the graduates on the left and right stand-up from their seats, then turn and walk towards each other. All their seats simply disappear between shots. Obviously done deliberately for the dance sequence, but a mistake nonetheless.
Deliberate mistake: When Penny is loaded onto the ambulance at the end, her gurney is rolled straight in. This is wrong, the paramedics will collapse the wheels and slide the gurney into locks to hold it still during the drive. It was done this way since Mittens and Rhino were hiding in the collapsible section.