Suggest a possible source of error that could account for an incorrect dating measurement using a radioisotope method.
HUMAN ERROR ONLY. If the scientist was careless, then perhaps the sample could be contaminated. But this would be a very vulgar and unprofessional error.
otherwise... radioisotope dating is exceptionally accurate and predictable. The decay rate of radioactive isotopes has been proved, re-proved and re-re-proved to be pure clockwork. It is amazingly accurate.