Well, roughly, carbon-14 dating systems are old. They were founded in 1948. They assume that the material being tested hasn't decayed itself, which is wrong, it does. So the dating compounds itself through decay, making it unreliable. In order to take decay into account, you have to make further assumptions that the decay RATE is the same which is also not true. The decay rate varies, further skewing the results.

Then they replaced it with radio-carbon dating which also saw similar problems with even further differentiating results and are now testing some other radio technology that I will have to go look up because I can't pronounce it or spell it.

You can go ahead and confirm the above though, because it happens to be true.

Also of interest to me was, that underwater specimens are completely off the charts of what was expected in tests within the SAME fossils from the same areas. They've now concluded that underwater specimens have much different decay rates as well as specimens that were exposed to water for periods of time.