Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the closure temperature. Changes in the earth's magnetic field from normal polarity to reversed polarity or vice versa. These rocks intrude even older rocks that have not been dated. Harland et al. If the age of this unit were not so crucial to important associated hominid fossils, it probably would not have been dated at all because of the potential problems.
Refuting the conventional geological time scale is not an exercise in collecting examples of the worst samples possible.
Given isotopes are useful for dating over a range from a fraction of their half life to about four or five times their half life. This means that occasionally the unstable isotope will change its number of protons, neutrons, or both.