At the end of some of the stages in Super Mario Land 2, you get to play a kind of skill crane game where the claw moves left and right over a conveyor belt and you have to time when it tries to grab a thing.

But, if you wait for it to go right and then all the way back to the left once, and press the A button

You'll grab the 3UP every time!

Which is definitely the most useful of the items in there.