Any time these cats showed up
The Lost Levels in general is just ridiculous, which also makes it one of the GOAT's in it's own twisted way
Poison Mushrooms that eventually become more common while actual mushrooms are hidden in ways that make them harder and harder to get
Hammer Bros that run up on you like
Having to find invisible blocks to clear certain jumps
....and invisible blocks keeping you from jumping up in certain areas

and in the case of one level while some aforementioned hammer bros are running up on you like
Impossibly long gaps that you can only clear by bouncing off a parakoopa's head at the exact moment
And in later levels having to clear jumps like that while Lakitu is flying around throwing stuff at you
....aaaaannnnd trying to clear jumps like that while the wind starts blowing randomly, affecting the trajectory of your jumps
Going through castle mazes with a fireflower like

...
until a gap comes through that you can't pass unless you're small Mario, which means you have to get hit on purpose... in the hardest game in the series
When you're like

when you find a secret path and a warp zone is revealed... that takes you back to the very first level

And there's a bottomless pit right next to it, like the game's telling you to commit that
Braving all of that craziness and finally beating the game (even worlds A through D) though were