If your dough is over proofed, it will have taken on more air pockets than it can structurally hold by the time it goes into the oven. It will often deflate before the crust and crumb can set resulting in a in volume, or worse case, a wrinkly mess.

What happen if proofing too long?

Over-proofing happens when dough has proofed too long and the air bubbles have popped. You'll know your dough is over-proofed if, when poked, it never springs back. To rescue over-proofed dough, press down on the dough to remove the gas, then reshape and reproof. (This method won't work for sourdough bread.)

What happens to over proofed dough?

An overproofed dough won't expand much during baking, and neither will an underproofed one. Overproofed doughs collapse due to a weakened gluten structure and excessive gas production, while underproofed doughs do not yet have quite enough carbon dioxide production to expand the dough significantly.