7:30: Playing this level is what convinced me that this is something I do often, even if I don't realize it. 18:03: The first red stemmed Piranha Plant obstacle in that room is much easier with the feather IMO. Would be worth learning to use it just for that TBH. I know that a fair number of people didn't like this level, calling it out as their least favourite level in the game. While I wouldn't go that far, I will say that I prefer levels that make me feel like a ninja overcoming difficult obstacles rather than a clumsy oaf who can't do anything right. IMO, this makes the level feel unfun to me, even if it's not unfair (as you noted).
4:04 daaaang! 4:36 not again!! 11:25 hooray life waster coin! 12:29 hello! :D 14:50 Two tile high holes: Big Mario WILL NOT get in there if you just hug the wall. Small Mario has a 50% chance to get in there, but ideally you just have to thread the needle for these type of holes! Duck-jumping while big optimally. a general thing that helps a lot in this level is ducking to stand still! Just be wary of momentum! That green switch is my 2nd fav level of the hack!
