A lot of people think this is a bug for some reason. He slides a little bit when you stop running (or when you land, continue running for as much as one frame, and then stop). It doesn't happen when you land from a jump without holding down left or right because there's no reason for him to slide if he only has vertical momentum. In fact, I can show you the assembly code that does it if you're interested.
The thing is, MM has precise control over movement in air while jumping so in order to reach a far away platform you have to hold down left or right and then INSTANTLY let go the very second he touches the ground or else he'll slide. This killed me on so many levels especially Wily's stage where you have those laser shooting moving platforms and there are spikes all over the place.
Also, Yellow Devil FUCK U (this was before i learned about the select/elecman trick).
Also also also Gutsman's stage with the dropping platforms. You have to jump perfectly here because MM falls the instant the platform touches the gap and the platforms move so slow that if you jump even a second too early you'll fall.
It's strange, I beat these games so easily as a wide eyed boy of 8 years but now as a grown ass man I can barely stand them. I should do a study in how vidya skill degrades with time.