What would happen if you added elements to fossils or whatever you are dating?
Throws the whole dating system out of whack!
Radiometric dating assumes that the radioactive elements you are measuring have not changed over long periods of geological time - aside from decaying at their half-life rate. If you added elements, this would result in faulty dates. This can happen in the natural rock cycle in that a radioactive elements gets re-set by say weathering and erosion. So, scientists have to be aware of this problem when they do age dating.
To counter this possibility, they usually do not rely on one age date alone, but will do multiple age dates on different samples from different locations if possible. They might also use different radiometric techniques (e.g. U-Pb and Rb-St) and see if the dates agree with each other - if they are wildly different, there is probably a problem somewhere!