I read in a biography that if someone was sentenced to death penalty and the execution squad misses when firing at him, he will be released as a free man. Is it true that there ever was such a law or was it ever practiced?
Cotton Mather (the son of the then President of Harvard University and a strong advocate of the Salem witch trials) recounted this case more or less contemporaneously, and there is no good reason to doubt the veracity of this part of his account.
I can find no reference to a law. Quite a few to occasions where a death sentence was commuted to life imprisonment or transportation, even to release at the discretion of the court, not as a legal right. One or two where the convict was returned for a later attempt at execution, presumably successful.
So, nice idea, but almost certainly not true.