Castle's mother, played by Susan Sullivan, is a long-time struggling actress. In the episode "Tick Tick Tick" she watches herself in the pilot episode of The Incredible Hulk, in which Susan Sullivan appeared.
Special Agent Shaw says the killer was extracting formaldehyde from embalming fluid to make "cyclonite, a near military-grade explosive". First off cyclonite, AKA RDX, is the prime ingredient in C4 and the most powerful of the commonly used military explosives. Secondly, he would have a lot of bother purifying and converting the formaldehyde into hexamine, which is then turned into RDX. Hexamine is sold by the pound at hobby stores.
The trivia item below may give away important plot points.
In one scene, upon learning Castle made breakfast for Beckett, Ryan and Esposito think there's something going on between them, in spite of Castle's insistence that it was a purely platonic gesture. Ironically, when Castle and Beckett do become romantically involved in season five, they're so good at keeping it a secret that Ryan and Esposito are almost completely clueless; in "Castle: Murder He Wrote (2012)", Ryan only finds out about their relationship by dumb luck because he badgers a murder suspect during an interrogation.