Factual error: I-131 (or any radioactive material) "at the end of its half life" does not instantly become non-radioactive, as was depicted in this episode. Radioactive decay is a continuous process; the half-life has no "beginning", but tells you how much time after "now" for however much is there at any time "now" to be reduced to one half that amount. Wait enough half-lives and the amount will fall to an inconsequential amount, but one is never enough
Factual error: Firstly, I-131 has an eight day half life. It is thus impossible that she is poisoned on day 1 (a dose enough to kill her), and then the following day there is no detectable rad. reading at the source. The eight day half-life also comes into play with the hand that becomes "neutralized" to radioactivity. Secondly, the discussion re: alpha, beta, and gamma radiation is true, however if breathed/ingested, I'd sure rather it be a gamma emitter than an alpha emitter, as alphas will deposit more energy (this is contrary to what was stated).
Character mistake: Horatio twice reads an extract from a letter threatening the life of the poisoned woman. The first time, he reads, "If you ever try to contact us again we will kill you," and the second time, "If you ever try to contact me again I will kill you." A close up of the letter shows it to read, "If you ever try to contact me again i'll (sic) kill you".