Myself and a friend went to complete Super Mario Brothers 3 in one sitting. We got to the final world and just couldn't figure out where to go in the castle. We tried literally every possible combination of things we could think of, including running frantically back through the level after hitting a Pow block thinking that maybe a hidden door appeared. Needless to say, we overlooked one of the most simple of solutions - keep moving right after hitting the Pow block. I to this day can't figure out how we managed it, we actually had to look up the answer online after spending the better part of 2-3 hours on it. Then we got to the final Bowser fight, and the console froze up. Not a word of a lie.