How often are serious injuries caused by epidural during childbirth?

Very rare: You very rarely hear about a serious complication during epidural it is a very safe anesthesia technique.

Very rare: The main concerns with epidural placement during pregnancy are bleeding, infection and the possibility of a headache. By a serious injury i take it you mean nerve or spinal damage. Here the location of the epidural below the termination of the spinal cord makes it virtually impossible to damage this organ.

Epidurals are : Extremely safe. With proper patient selection the risk of serious complications is extremely low.