Executive Meddling: The infamous "train duel" scene was filmed in a rush after the director was fired over creative differences. Also, in the near-final script, Agent Markov is revealed to be the Big Bad who wants to replace the President-to-be with an Evil Doppelganger loyal to him, 47 is chased by Spetsnaz (not the other clones) and the whole Evil Plan is foiled by Mike (who recorded Markov's Engineered Public Confession after slipping one of 47's spy microphones under the collar of his coat and gave the tape to a Russian general) and 47 (who manipulates Markov into ordering his Dragon to shoot the President during Udre's funeral and blows him up anyway).
Money, Dear Boy: Timothy Olyphant has admitted in interviews that he did the film as a means to help pay for his house after Deadwood was canceled.
The comic book:
Screwed by the Network: DC became notoriously gun-shy about promoting Hitman after the Columbine High School shootings in 1999, which is one of the factors that ultimately led to its cancellation.
Throw It In: One time the writers were coming up with the silliest ideas for comic characters and when Dog Welder was suggested the contest was over. Lol and behold, the character was made up.
The video game series:
Executive Meddling: This is the reason why there was such a long gap between Blood Money and Absolution. Apparently the developers started work on a new game for release in 2009 or 2010, but Eidos decided that the Kane and Lynch franchise (also developed by IO Interactive) was going to be the next big thing (it wasn't) and ordered them to produce a sequel to that game instead. Square Enix on the other hand were far less in love with Kane and Lynch, and upon buying out Eidos they put that franchise out to pasture and let IO get back to the Hitman series.
No Export for You: An odd version. Contracts was unavailable on Steam despite all of the other Hitman games being released on the platform due to a music licensing issue. It has since been added in the Steam store in January 2014.
The Other Darrin: For Absolution, the original voice actress for Diana (Vivienne McKee) was replaced by Marsha Thomason (best known for her roles on LOST and White Collar).
Agent 47 himself nearly got this in Absolution too, when his voice actor, David Bateson, was revealed to have not been contacted by IO. Following many fans protesting/begging/protesting and begging at the same time, Bateson was called back and did the voice work again in Absolution.
Throw It In: According to Word of God, Absolution's Instinct mechanic began life as a developer tool to determine pathfinding after the AI became so insanely complex they could no longer playtest properly. It was later added to the game proper, provoking much outcry, but it doesn't do much in higher difficulties other than helping you blend in.
Keep Circulating the Tapes: The show hasn't been seen since its cancellation, primarily because the "fact videos" were licensed for one showing each and Wolpert hasn't been able to negotiate new terms.