Why isn't radiometric dating always acurate?

Question

Why isn't radiometric dating always acurate?

Answer 1

More generally: Radiometric dating is a mathematical approach that only applies to the real world when several assumptions about the real world are met.  When these assumptions fail to be met — through contamination or otherwise — then the math ceases to accurately represent the real world, and so the method necessarily fails.  

Assumption (1) below has stood through many experimental trials and is understood to be a law-driven physical process across all scenarios and timescales.  The other assumptions need to be assessed for each particular case and each particular system being analyzed.

The assumptions:

(1) the radioactive element decays at a constant rate

(2) the rock crystal being analyzed is not contaminated by infusion of excess end product

(3) the rock crystal contained no end product (or a known amount of end product) when it was formed

(4) leaching of the parent element out of the rock sample did not occur.

Answer 2

Contamination of the sample.

Radioactive carbon 14 is not produced at a regular rate.

Massive amounts of old carbon can be released by volcanoes or methane deposits etc.