Ever since the sediment caused by the eruption of Mt.St. Helens was carbon dated to millions of years ago, I simply do not believe carbon dating is anywhere near accurate.
Since carbon dating only works back to about 50000 years, what you described has never happened.
Carbon dating is only valid for less than 50,000 years - there are other methods of dating past the 50,000 year mark. Like for volcanic eruptions argon 49 decay test.
Anyone who claims to use carbon dating is putting you on.
Dating ancient sediment involves using radiometric methods based on the natural radioactive decay of certain elements such as potassium and carbon. Geologists also use other methods - such as electron spin resonance and thermoluminescence.