In Episode 1: The Road to the Big Windmill, if you skip the Sun Sprite after you defeat the Polluted Piranha Plant and go up the windmill you can battle Petey Piranha. This is the only episode in the entire game that can be skipped. Also, on the way up the windmill a Pianta will say that you are getting ahead of yourself, this is a reference to Super Mario 64 where you can get Power Stars which are not necessarily Mario's current objective.
Contributed by Outofmind23
Video