Radiometric dating problems

Processes or Time
To date rocks or other objects, scientists typically use radiometric dating. In short, the ratio of radioactive and stable isotopes in the sample are determined and the measured rate at which the isotopes decay is used as an indicator of the age of the sample. However, it is typically unknown and simply assumed whether these ratios of elements are the result of radioactive decay over time or other processes that have taken place in the rock.

Uinkaret Plateau
Ages of Rocks in Millions of Years

Sometimes different methods used on the same rock produce different ages. Furthermore, the same method can produce different ages on different parts of the same rock. Sometimes these are close but other times they are very different.

Isotopic Fractionation
Isotopic Fractionation is a physical separation of isotopes and a non-radioactive source of isotope ratios. It can be caused by heating and cooling, water flow, contact between high and low concentration magma and just normal molecular motion. Evidence for Isotopic Fractionation does show up in isotopic data so it is a factor that needs to be considered.

Water flow through rocks is important because all parent substances and many daughter substances are water soluble. This is particularly important in light of the Biblical flood.

Formation of sample
How a rock is formed is important to understanding its isotopic make-up and any dates derived. The isotopic make-up of original material is important, as is mixing of magma with surrounding material. The conditions of formation are also important, because both the cooling rate and the opportunities for mixing affect isotope ratios. Quick cooling or not having contact with the air can affect theoretical mechanisms for "resetting" the clock.

Anomalous dates
Some times radiometric dating produces impossible results.

Uranium-Thorium-Lead Method Ages" in Billions of Years

Some soil from the Moon has been dated as more than a billion years older than the uniformitarian age for the Moon. It was explained by the processes of heating and cooling that the soil had been through.

Some rocks dated older than the 4.5 billion year evolutionary age for Earth.

Recent or young volcanic rocks producing excessively old K-Ar "ages": From: Potassium-Argon and Argon-Argon Dating

Examples of negative ages
From: G.B. Dalrymple, “40Ar/36Ar Analyses of Historic Lava Flows,” Earth and Planetary Science Letters,6 (1969): pp. 47-55.

Some rocks have been measured with negative radiometric ages, in some cases in terms of millions of years. Isochron dating can also produce negative ages, by producing a negative slope. K-Ar and Ar-Ar can result in negative ages when atmospheric argon is considered. So if these are real dates then you can hold a rock in your hand that won't form for hundreds of thousands or even millions of years yet.

Now in all fairness Ar-Ar dating can get the right age for a sample of known age, but it can also date samples as way too old, but without a known date there is no way of knowing when it is too old. One key factor is the fact that Ar-Ar dating needs a standard of "known" age. If the standard is of historically known age, such as would likely be used for testing Ar-Ar dating on sample of known age, then one would be more likely to get the correct age. For allegedly older samples K-Ar is used to "date" the standard and as such it still has the same problems as K-Ar dating.