While the small variations in isotope decay that have been reported may not invalidate all isotopic dating, they raise questions about the assumption of completely uniform decay rates.
A second assumption is that the sample being dated has not experienced any loss or contamination of C over its history.
These effects are corrected for by comparing samples from different locations.
The reasonableness of this assumption probably depends on the environment around the sample.
A sample that is sealed from the surrounding environment is more likely to avoid contamination or loss than one in an open environment where materials may be carried into or out of the sample by water or simple diffusion.
Carbon-14 dates usually appear to be reasonably accurate whenever they can be checked against historical records.
For example, when the Dead Sea Scrolls were dated, three methods could be used: 1) Dates written in the documents themselves (like the date at the start of a letter) 2) Paleography, which uses the style of script used to write documents to date them, and 3) Carbon-14.
Carbon-14 dating cannot be applied to materials that have no C dates are less than that figure.