Go to level 2-5, then go to the very end of the level until you get to the boomerang throwing turtle, kill him and then fall into the quicksand on purpose, then keep holding (right), and rapidly mash (b) until you jump by the steel peice and keep on falling down until you and jump to the right and there should be a jumping block dude,(don't kill it), jump on top of it and it will jump there will be a star coin get it,then go to the top again and get the flag.

Congratulations youve got the secret star coin.