In Episode 2x02 - The Hounds of Baskerville our Mr. Holmes is - once again - bored and wants a case:
Sherlock: I NEED A CASE! [...] What am I saying? This is brilliant. Phone Lestrade, tell him there's an escaped rabbit.
John: Are you serious?
Sherlock: It's this, or Cluedo.
John: Uh, no. We are never playing that again!
Sherlock: Why not?
John: Because it's not actually possible for the victim to have done it, Sherlock, that's why!
Sherlock: But it was the only possible solution...
John: It's not in the rules!
Sherlock: Then the rules are wrong!
Having never played the table top game (presumably that is what they are referring to) myself, I am unsure if that is a valid (and known) criticism of the game's rules (Google doesn't seem to think so: "Cluedo rules wrong" basically leads to quotes from the episode or are sites telling you that most people play the game wrong because they misunderstand the rules afaict) or if it is merely a throwaway gag from the writers?