Dough that's been fermented too long -- the gluten network has broken down, gas has escaped, and the dough won't hold its shape or spring in the oven.
Signs of overproofing: the dough is extremely sticky, it spreads flat instead of holding its shape, it doesn't spring back when poked, and you might see the surface looking deflated or pocked. The crumb of overproofed bread tends to be dense with tiny, uniform holes -- the gas diffused out before baking. If you catch it during bulk fermentation, your best move is to shape it and put it in a loaf pan. The bread will still taste good, just with a different texture.
