When discussing an answer that I provided in another question (Battlestar Galactica: How to balance core game), the question arose of whether it is mandatory to play at least one card in skill checks. The rule book from the base game says:
Starting with the player that is Play Skills: to the left of the current player (and ending with the current player), each player may play any number of Skill Cards from his hand facedown to the common pile.
So, what does "any number" in this context mean? I assume that 0 would not be "any number", since playing 0 cards is not playing any card at all. I saw in this question (Strategy for succeeding at Crisis skill checks) that people generally assume that 0 cards can be played, but I think there is some flaw in the statement that makes it dubious. In the Revelead Cylon case, the book is very clear and states that a Revelead can opt for not playing cards in skill checks.
In the end, what is the rule for human players? Must them play at least one card or not?