I don't remember the stage number, so don't read any further unless you want a description of the level.
in the level where you have to ride the for the second half, there is a platform right before the end of the level where there is a yellow pipe at the top of the screen.
there is a stack of coins leading up into it, propeller suit required.
you can see these coins from the pipe exit at the end of the stage.
it is also in the gameplay videos if you wanna see one way to get up into it.
I found it before the video, but I watched the video before I actually made it up into the pipe. I did not get up in the pipe the same way as in the video.
I have no idea what this all meant now, because i got to 7-6 via (this is getting annoying..)
world 7's fortress completely by accident. I kept having a hard time during the second half of the level so i gave up trying to get the last star coin (the one behind the blocks). When i went back i was able to get it, but the platform wasn't in my reach, so i jumped to my far righ trying to get to another landing, and after doging a few enemies in mid-air, stumbled upon the exit. No yellow pipe (that i was aware of) and no need for a propeller suit.
With that said, there is one thing thats been bugging me about W7's Castle, and maybe i just don't have the hint movie unlocked for it yet, but even though i have all the star coins.. what the hell is that exit pipe for right before the boss door? After the long run of spiked pillars, when you arrive at the fat hammer bros. gaurding the door, you can jump up to the right of your left hand wall and hit a hidden power-up box containing the propeller mushroom. If you hover up in that same area, you'll push the clouds away and discover a green pipe, but you can't go in it. I've gone back thru as much of the stage as i could with the propellar suit hoping to find some secret i've missed hidden in the clouds, but theres nothing! It really irks me!haha just beat world 7-6 on my first try, and got all the star coins. I loved it! guess i found out what that pipe was for

One more question though... whats up with mario going sans hat? i got 99 lives for the first time, i'm betting thats it..