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.
Critical Research Failure: Agent 47's eponymous Silverballers are said to be modified AMT Hardballers in .45 ACP like they are in the game. In actuality, they are 9mm Para-Ordnance P-18. This is probably justified by 9mm blanks being easier to adapt than .45ACP.
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.