How do you stop bread from cracking?

Always shape your loaf on a piece of non-stick, silicone paper, then when it's proved, stick it straight onto the hot stone and reduce the temperature of the oven down to around 180-200°C. Most loaves are soggy at the bottom because they've not had the heat but you can simulate that at home by using a hot stone.

What happens if bread is over proofed?

When a loaf proofs for too long, or is proofed at too high a temperature, the dough over-aerates and the gluten over-relaxes, allowing the gas pressure inside the loaf to overwhelm the dough's internal structure.