Toward the end of filming, Zac Efron had appendicitis. At first, Thomas Lennon advised Efron that it was, in fact, bad wind and advised him to hang upside down to cure it. What started out as a stomach ache, soon led to him checking it out after filming and having surgery the same night.
Zac Efron called Matthew Perry for help on his lines and mimicking some of his movements to give a more authentic performance.
Visual effects were not used when Zac Efron does the basketball tricks during the cafeteria scene, he really did accomplish them on his own.
In one scene, Mike wakes up and begins describing his "dream" of being in high school again only to find his daughter, Maggie, caring for him. This is an homage to the counterpart scene in Back to the Future (1985), in which Marty McFly wakes up and finds his teen-aged mother caring for him.