The Goombas in the original Super Mario Bros. games have kind of a rough existence. All they can do is walk slowly toward Mario in a not very menacing way. Even rougher is that there's a glitch (or maybe it's a feature?) that lets you kill them twice. This is actually kind of tough to pull off.

First, find a place where there's a koopa in a shell, something for it to ricochet off of, and some Goombas. Like in World 3-2, for example.

What you have to do is kick the shell away from you, then quickly start stomping Goombas.

On its way back stomp some Goombas, and if the shell collides with one of the flattened Goombas, it'll get killed again. Brutal!

To explain it a little better, I've created a short animated .gif here to show it in action.

Man, sucks to be those guys.