How common are cracked toilet tanks? And why would they crack under normal conditions

If mounted to wall, mounting bolts are to tight causing pressure on tank.

Where is crack located. More info please.

Chuck

Most toilet tanks are creaked because of faulty installation. Let me explain. Here's a cutaway of a toilet tank, (see image). Notice the tank bolts that secure the tank to the bowl.
If the bolts are pulled down tight against the bowl when someone leans back it puts a strain on the tank base at the bolts and the china will crack at the bolt holes.
The correct way to install tank bolts is to leave at least 1/4 inch of play in the tank instead of cranking it down tight. That way the tank will "give" a bit when someone means back. Hope this explains it to your satisfaction. Cheers, Tom