Don't tell my dog that when I call him with a treat, I'm going to reach out quickly and grab his collar so I can get him in. (It takes several tries. He's quite wily.)
Don't tell my students that when I play kickball with them, I always "bunt." (I can run fairly fast, for a short distance, but I can't kick worth a d***.)
Don't tell my hair stylist I'm going to schedule a hair-cut in the next week or so... If she gets advanced warning, she might call in sick. (It's been a loooong time since my hair was dipped, and parts of it are as gray as a rat's back.)
One of my friend's family had a "seven year" rule. Whatever trouble they got into, they couldn't tell their mom about it until 7 years had passed. Their mother figured, if they weren't in prison or dead or hospitalized, it must have not been too serious. Some of the boys in the family actually noted the exact day they could tell their mom, because it was that good of a tale.
What story could you begin with "Don't tell..."?