Revealing mistake: Season 5, episode 18 "Point of No Return": When Danny fires at the bad guy, there are sparks coming from the metal pipes when his bullets hit them. One of the things causing the sparks burns too slowly, taking over a second before it fades out.
Continuity mistake: An assistant shows Craig T Nelson and Gary Sinise a mockup of a New York Ledger news page, with a photo of an NYPD officer and a headline that reads: line 1 - "NY's", line 2 - "Finest take", line 3 - "A break". When Sinise takes the photo and puts it on the desk a few seconds later, the text layout is on 2 lines instead of 3.
Factual error: All the victims were supposedly killed using thalium-201 - a radioactive isotope with a half-life of just 72 hours. e.g.: In the space of two months there would be 10 nanograms for every gram applied to the source of the radiation poisoning (i.e the book) - not enough to make Sid ill without making the perp very ill. He would have had a much higher dose than anyone infected.
Add timeAndy Benham
Audio problem: Season 5, episode 17 "Green Piece": When Stella is typing on the Apple keyboard she hardly lifts her fingers from the keys, yet we hear a loud clicking sound. That type of keyboard doesn't make that kind of sound, unless you pound the keys with great force. Also, the clicking sounds don't match up with her keystrokes.
Plot hole: At the end of the episode, Adam and Stella are checking how many others there are bearing the same name in a government database. "Stella Bonasera" gives one result, being the Bonasera character. A search for "Sid Hammerback" also gives one result, a 90 year old woman (the characters say it has to be his mother). But Sid himself should have shown up too, with the search yielding two results, not just one.
Factual error: The episode has a character in it called Klaus Braun ("Braun" being the German word for "brown"). However, all of the characters pronounce his surname wrong, saying the "au" like the "or" in "bored", not the proper German pronunciation, which would in fact, be the same as the English meaning, "Brown". Other German character's names were pronounced correctly, and surely Klaus/Abraham would object to having his name pronounced incorrectly.