The account was reviewed by a Medical Examiner who traced through the trauma inflicted on the person of Jesus. From the beard pulling, beating, being struck with a Roman "flagellum" 39 times, being marched across Jerusalem and back three seperate times, and going without water or food for 18 hours prior to being even nailed on the cross, the conclusion reached is that Christ was in critical condition before even being nailed on the cross.
The crucifiction account itself has no historical flaws. The thieves needed to have their legs broken (to hasten death by suffocation) because they were not bleeding from a massive back trauma and in Hypovolemic Shock. The "Blood and Water" reference in John's account is consistent with Pulmonary Edema, brought on by Hypovolemic Shock and the Carbonic Acid buidup from being unable to properly exhale.
Finally, understand that the Romans that actually did the execution were well schooled in what dead people looked like, and that the usual penalty for a botched execution was replacing the intended victim with your incompetent self. The Romans would have made sure that Jesus was dead; hence the coup de gras administered with a spear driven into the heart.
He was dead alright. See the Case for Christ by Lee Strobel for additional (gory) details...