Without giving away any specific details, I personally counted about 41 mistakes. Some of them were very subtle and may likely be missed by even the most loyal of friends and true! I'll have to watch it again to see what else I pick up on, because there were at least a few instances where there were multiple gags within moments of each other, and I'm sure I missed something.
Like sideshow, that's probably the most attention I've ever paid to little details during the show for the entire hour. Even with the April Fools' Day gimmick attached, this one was a lot of fun to watch. Huge props to the show for putting this one together and not making all of the changes painfully obvious -- I wouldn't doubt that when they unveil all of the gags next week, no one will have every single one of them on their list!