Amazing beliefs

 What would happen if someone survived being sacrificed in prehistoric societies where human sacrifice was common?

It was frequently difficult to survive the sacrifice because there were many individuals present to ensure that wouldn't happen.

The Geto-Dacians, a subgroup of the Thracians who lived in a region roughly equivalent to modern-day Romania, are the only society I am aware of in which the survival of the man prepared for sacrifice might occur (at least for a particular form of sacrifice).

They would select a young, fit, attractive, clever, and well-bred man every few years (from among the sons of the nobility) to represent the Dacians before the gods. After hearing the message, he would ritualistically be hoisted onto three spears by warriors.

The message would have been conveyed even if he were to pass away. But if he survived the spears, he was deemed unfit and shamefully exiled from his village.

Basically, the Dacians were devout Christians who believed in the afterlife and saw this world as a "Valley of Tears," mourning births and celebrating deaths (well, according to Herodotus, and he's known to stretch the truth occasionally - the Dacians were probably more moderate than depicted in that regard).

Returning to our potential sacrifice, if he survives, it would imply that the gods deemed him to be an unworthy representative who was not deserving of eternal life and was therefore sentenced to remain in this world for a bit longer (how munch longer probably depended on the gravity of the wounds inflicted by the spears). This implies that a new envoy would be chosen each time until one "good" enough (defined as dying on the first attempt) was found to carry the message.