Toilet overflow...how?

I'm not a plumber so this question may be totally stupid, but...

Last night my wife came home to a flooded bathroom. She said water was coming from the tank of the toilet not the bowl. How is this possible since there is a drain in the tank that will not allow the water level to go above it?

Normaly the over flow tube drains water unless...

#1 the toilet is clogged past the point where the over flow tube drains.

This normaly will fill and over flow the bowl.

#2 the incoming tank water flow is faster than the over flow will take it out.

What would clog or slow down the drain in the tank? I can't even imagine what would have caused this to happen.

If the fill valve sticks on... and the overflow tube is not cut off at the right height, of the incoming water is coming in faster than the water can go out through the overflow the tank can overflow.