There is a tendency for toilets to crack. The good news is that you don't have to worry if your toilet just cracked.
It happens all the time and there are affordable ways to fix it. It is common for people to crack the lid of the toilet tank when replacing it.
Infrequently, the lid of the toilet tank hits the toilet tank as a result of its weight. It is more vulnerable to breaking than the lid, so the toilet tank will simply crack.