Contamination is a huge threat to the accuracy of radiocarbon dating. This can happen when samples pick up the wrong carbon from packing materials, are exposed to atmospheric radiocarbon or experience cross-contamination with other samples.
Scientists previously believed that the ratio of C-14 (radiocarbon) to C-12 is constant, but UCSB states that other dating techniques show the ratio has changed over the course of Earth's history. The ozone layer has weakened and caused radiocarbon levels to increase over the past 20 to 30 years. This significantly impacts the accuracy of carbon-based dating.
Recent samples usually have not decayed enough for an accurate reading. In addition, Dr. Tom Higham at the University of Oxford says that the age of artifacts more than 50,000 years old is difficult to pinpoint, as the samples no longer have enough radiocarbon for accurate dating.