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.
![](../sites/default/files/goomba1.jpg)
What you have to do is kick the shell away from you, then quickly start stomping Goombas.
![](../sites/default/files/goomba2.jpg)
On its way back stomp some Goombas, and if the shell collides with one of the flattened Goombas, it'll get killed again. Brutal!
![](../sites/default/files/goomba3.jpg)
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.