uh oh

Well at least you didn't 100% it twice like me... that just shows I had nothing better to do at the time.

WTF is the megaflash infinijump sticker!?!?

To answer your question, @Gjlm its a super rare sticker found at the end of the 5-3 secret route, just head right from the comet piece.

OK, well im on 2-3 and i have no idea what to do

That level is a bit of a maze. You'll need to find both the exits (one is only available after falling down one of those little switches for the first time). Also I won't say what in case you wanna figure it out yourself, but there's a Thing here which will come in handy later.

Also I'll probably make a new strategy guide post if you have more questions so Skullaurus isn't blown up with replies, just check back in this community in a few minutes.