Character mistake: Horatio writes the symbols for sulfuric acid, H2SO4, on a clear board. Both numbers should be subscripts but he writes the 4 as a coefficient (which precedes the name of the acid but he writes it after), suggesting a lack of basic chemistry knowledge.
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".
Answer: Earlier in the episode, Horatio tells Mac that Penrod was seen in Tribeca getting his leg set, only to bolt when he learned a nurse wised up the cops. Caine and the NYPD probably got Penrod's address from the hospital.
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