There are some materials that can cause major problems if you try to grind them. In particular, the big no-no with grinders is aluminum. Aluminum melts easily, and if you were to try to grind down aluminum, it would start to melt and the aluminum would coat the wheel.

The stone would grind the aluminum for a while, but particles would start getting embedded in the pores of the stone. And the next time the stone comes around, there's no grit exposed, only little bits of aluminum. So the aluminum on the piece rubs against the aluminum on the wheel, producing more heat, which produces more melting, and so on.

The natural response of the person at the wheel is to push the piece harder against the wheel. But this will only cause more friction and heat up the wheel faster. If the wheel heats up enough, it can explode.

Grinding aluminum

If someone does grind aluminum on a grinding wheel, causing it to be coated in aluminum, it doesn't mean the wheel needs to be thrown away. But you will need a dresser, which is a hand tool with two little star-shaped wheels on the top. The wheels are made of fairly soft steel, and are much softer than the stone. When you hold the dresser against a spinning grinder wheel, the tips of the stars act like little hammers, gradually wearing down the wheel and knocking loose all the aluminum.

Dressing the wheel