Possible reasons that the Carbon 14 testing was wrong:
1. Invisible patch hypothesis
Comment: The invisible hypothesis is the most discussed idea in this thread. The evidence against it is daunting to the point that for any practical purposes it is reasonable to accept that the invisible hypothesis has been proved false.
2. Bioplastic coating
Comment: Roger Sparks, a carbon dating expert from New Zealand, makes very convincing arguments against this possibility here:
http://www.shroud.com/c14debat.htm. This is clearly a fringe theory and bioplastic coating is not a recognized source of error for carbon 14 age dating. Even if it bioplastic coating was shown to be a possible source of error for the dating of a fabric the possibility that it could cause anything like the 1300 year error hypothesized for the shroud dating is essentially impossible for the reasons that Sparks outlined in the discussion linked to above.
3. Collusion
Comment: That are many hypothetical permutations of collusion by the people involved in the sampling and testing. All of them seem to be extremely unlikely. For one thing nobody has shown any kind of possible motive. But even if a motive could be theorized the well documented sampling and testing procedures coupled with the use of controls seems to preclude it.
4. Effects from the fires that damaged the shroud
Comment: This is a theory that carbon from the burning migrated into the shroud and perhaps there is a mechanism whereby carbon 14 as opposed to carbon 12 and carbon 13 preferentially migrated into the cloth. Sparks in the link above, argues both that preferential migration of carbon 14 from a fire is not possible and that if there was a problem with carbon 14 dating of samples based on exposure to fires that it would have been noticed by now. Meacham argues that there is very little if any carbon 14 dating that has been done on a sample alleged to have been a first century artifact that had been subjected to a fire in about 1500 AD so no conclusion is possible about the carbon 14 dating on the shroud. The problem, of course, with Meacham's line of argument is that carbon 14 dating might be deemed unreliable in almost any situation because there is always going to be something unique about a sample. No scientifically recognized reason has been put forth as to why the nature of the shroud's history would cause an error remotely in the range of the error hypothesized.
5. Other unknown sources of error for the carbon 14 testing
Comment: The argument here is that there are various known sources of error for carbon 14 testing and maybe not all sources of carbon 14 test errors may be known and with regard to the shroud one of these unknown sources of error has caused a test error.
This at best seems to be a remote possibility. None of the known problems with carbon 14 testing seem to apply here. The known problems include that the source of carbon for marine organisms is not atmospheric, some snails have been shown to incorporate older sources of carbon in their shells, and the use of old carbon sources in the preparation of materials as in the use of asphalt in some of the compounds used by the Egyptians in the creation of the mummies. One thing to note about the problems listed above is that they all have the effect of making a sample test older than it is and with the shroud the opposite problem is hypothesized.
