This and many other stupidly precise tricks, like pixel-perfect jumps against the sides of walls or a plethora of bugs involving Yoshi and his tongue.

The most common use of this particular bug is what’s shown in the image, carrying a shell and a key at the same time. You need to throw the shell against a wall while falling and land on it to extend your jump past what’s normally possible, while also carrying the key across the gap. It’s everywhere in Kaizo runs.