The Thunkit
Every game designer dreads the thunkit. This is a rule that you thought you included in your game, but didn’t. You didn’t write it in your manuscript-you merely thunk it.
This is why you need outside playtesters. As designer, you don’t always check your own rules text when a question comes up during play. To keep the game going, you answer it the way you thunk it, not the way you wrote it.
Trouble occurs when your outside playtesters infer a rule, rather than noticing that it’s missing. The more basic the rule, the more it follows the common assumptions of similar games, the more likely it is to by-pass the notice of your outside playtesters and make it into the finished product-or, rather, fail to appear there.
I’d tell you about a recent thunkit that turned up in one of my recently published games, but I’m too embarrassed to mention it.
Far more friendly is the Reverse Thunkit. Here you make a note during playtesting for a change you need to make, then return to your manuscript, only to discover that the rule you thought you needed has been sitting there in the manuscript all along.
I picture the Thunkit as a fuzzy Seussian villain, perhaps related to the Wickersham Brothers.