Hangman is the first episode of the sixth season. It was released on 25 September 2009.
The most common Hangman reference is to the spelling game, and its step-by-step elimination process, but in this episode, Charlie describes the "Unexpected Hanging Paradox", in considering that the assassin might be giving false clues to manipulate the expectations of the investigators and thereby change the timeline and the outcome.
While trying to figure out how to stop a shooter, Charlie asks the agents if they "remember when we used covering sets". While it might make sense for him to expect Don and David to remember, it makes no sense for him to expect that of Colby, given that the case in question was from Episode 4.01 "Trust Metric", in which Colby was not a member of the team but the target of the search.
[This appears on the beginning of the episode] 4 letters 400,000,000,000 stars 718 meters per second 60 stitches