Real-life example: "When I was a teenager, I was invited to a surprise sweet 16," says a New York woman. "A few days before the party, I excitedly asked the surprise honoree, 'Are you going to your party on Saturday?' Oops! I then begged her to not reveal my gaffe to the party givers, for fear they would ostracize me. This was in 1960. To this day, I cringe at the mention of a surprise party."

How to remove your foot: You can't stuff the cat back into the bag, but you should handle it with care once it's out. "If you collaborate with the person for whom the party is being thrown, you won't ruin it for the host, too," says author Peggy Post. If the host finds out the surprise is blown, however, Post recommends admitting to the mistake and doing your best to make up for it, either by helping to get ready for the party or by sending the host flowers.

In the future: The closer you are to the surprisee, the easier it is to slip. Try to limit contact with the guest of honor as the party approaches, and have a credible and airtight excuse prepared if she asks about your plans for that evening (out of town visiting your parents, dining with a friend she's never met, etc.).