One of the common problems with radiometric dating is that it is possible that the sample already contained some daughter element when the sample first formed, and this will throw off the measurement.
Well, it's a concern; it's not much of a problem. We can use a system like K-Ar in which the probability of initial daughter is low, although we don't do that much anymore. Or we can measure a rock like zircon, in which physics guarantees essentially no initial daughter. Or we can use an isochron method which is unaffected by initial daughter.
This is a common problem with the uranium/lead method, though it is often possible to make realistic estimates of the amount of orginal daughter material.
Er, the vast majority of U-Pb analyses are performed on zircons, for several reasons ... one of which is the fact that zircon
stronglyrejects lead at solidification so, to a very good approximation, the daughter product is zero. Of course, geochronologists nowadays are looking for such precise ages that a first approximation isn't good enough, and a correction factor based on measured non-radiogenic lead is applied.