In Super Mario Bros. 3 the power ups that come out of the ? blocks will most of the time go to the right.

But occasionally they'll go off to the left

And if you're not prepared for it, you might lose them. But, why do they occasionally go left in the first place?

Take a look at the ? blocks

Where you hit the block determines which direction the powerup will go. Hit the left side and it goes to the right. Hit the right side and it goes left.

You can use this to force the powerups to go where you want them, instead of possibly off into a pit.

This also applies to any of the 1up mushrooms and starmen.