Hilarious April Fools' Prank on Students

This is an applied statistics class at a university in the United States (taken mostly by biology students) that is expecting to review for their exam in the next class period. The person that comes in and teaches is not their teacher (obviously). He is teaching advanced probability theory to biological science students. The actual instructor was hoping one of the students would leave the room so that he could yell "April Fools!" Watch to find out what actually happens instead.
Donnie made a couple errors in his derivations. Can you find them?
Q: Who is filming?
A: There was a fellow instructor filming with a camera on a tripod. I told the students in the previous class period that I was being observed by someone in the department and it was department policy to tape the observation.