This terribly sad BBC news report documents how a family cared for their car-crash victim daughter for weeks, only to realise her identity had been mistaken as she emerged from her coma.
The casualty was in fact the daughter's college friend, meaning the daughter had in fact died in the crash and been buried by the other girl's family. The two girls look remarkably similar (see pictures on the original story link) and the coma victim was reported to have facial lacerations as well as wearing a neck-brace.
The family documented their experiences by web-log which can be found here. There is no explanation provided by the hospital other than the victim being wrongly identified.
[Thanks to Farrah for the links.]