Carbon dating?
Carbon dating has trouble with extremely young samples because of the effects of the atomic testing.
Also the range (± or plus/minus if the symbol does not come through collectly) can eat you up quickly. The date is usually figured at two sigma or twice the range in either direction; thus a date of 100 ± 50 has a range of 200 years at the two sigma (95% confidence) level. Once you get a few hundred years old, or if you run lots of samples, this can be less of a problem.