The Great April Spawn Explosion was an April Fools' joke planned by Zack to explode the entire Welcome Center. It occurred in Episode 75, on April 1, 2011. Zack blamed Mac for the explosion and she acted like it was her that caused it. He then made another video explaining that he made it all up. He had also made a save file of Scottland.
This was probably the biggest cause of the running gag "Ban Mac". Many viewers thought this was legitimate and therefore the final straw of Mac's destruction.