Is this common? What causes it and is there any way to ensure it doesn't happen?
Buildings can crack as things settle, in a way that just looks unpleasant & has abolutely no effect on structural integrity.
Buildings can also crack because they're built totally wrong, and they're about to fall down & kill millions of people.
It's a lot easier for builders to claim to just not cover any form of "cracking" ... but I suspect that if you've got a case of the latter (including proof from an engineer or external builder or whatever) than you've got cause for complaint to a regulatory body.