The shroud allegedly was in a fire during the early part of the 16th century and, according to believers in the shroud's authenticity, that is what accounts for the carbon dating of the shroud as being no more than 650 years old.
To non-believers, this sounds like an ad hoc hypothesis. The suggestions that modern biological contaminants were sufficient to modernize the date are also ridiculous.
It has been noted that if the shroud were really wrapped over a body there should be a space where the two heads meet.
It has also been noted that there is a space where the front and back of the head meet, and that what appears to be the outline of the back of the head is a water stain.
A weight of 20th century carbon equaling nearly two times the weight of the Shroud carbon itself would be required to change a 1st century date to the 14th century (see It may interest skeptics to know that many people of faith believe that there is scientific evidence which supports their belief in the shroud's authenticity.
For example, it is claimed to be the negative image of a crucifixion victim.
It is claimed to be the image of a man brutally beaten in a way which corresponds to the way Jesus is thought to have been treated.