When you give in to some sort of temptation (to purchase, to look, etc) and comment on how you "Failed your save" (or "failed your save vs. purchase" etc...
Whenever a certain group I hang out with catches one of the members just openly ogling an extraordinarily attractive gi...chi...er...woman, we often say that he "failed his save vs hot." (Although, if it involves peeking over an obstacle like the back of a diner's booth, we call it "prairie dogging.")
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...when you see the license plate "500HP" on the back of a sports car and wonder just what kind of monster that driver was thinking about...