I'm looking for an expression that's used to mean that you'll continue to teasingly remind someone of an incident after it has happened. It's not "I'll hold you to it," but is something just as commonly used. I've used this expression before but I can't remember what it is!
You are not going to live this down
If you say that you will never live down something bad or embarrassing that you have done, you mean people will not forget it Three million people saw the singer fall off the edge of the stage. He'll never live it down. I'll never live down the fact that I spilt champagne down my boss's trousers.
never live down. (n.d.) Cambridge Idioms Dictionary, 2nd ed.. (2006). Retrieved March 18 2017 from http://idioms.thefreedictionary.com/never+live+down
You might be thinking of holding it over someone's head.
hold over someone's head (transitive, idiomatic) To harp on; to remind continuously (especially of a misstep or defeat) I get one parking ticket and he holds it over my head for six months. - wikitionary
A situation similar to the one you describe arose just yesterday and became sufficiently popular (or notorious) that the print New York Times wrote a story about it: "Bouncy Children, Furtive Wife and the Family Blooper That Stole the Interview," renamed for online consumption at nytimes.com ‘BBC Dad’ on Going Viral: ‘This Is Now the First Line in My Obituary’ (March 15, 2017).
As you can see in the second of two video clips embedded in the story, the foreign policy expert/ talking head's home office was suddenly and unexpectedly invaded by his happily flouncing, very-much-at-ease four-year old daughter and then by his infant son in a baby walker, and then by his late-to-the-scene wife on all fours—all in the midst of a Skype interview with a BBC reporter. The daughter is clearly the star of the clip, although the furtive wife is pretty charming, too.
As the Times's online headline spoilingly reveals, the expression that the embarrassed expert dad later used in assessing the episode was
"This is now the first line in my obituary, right?"
—a line that you can easily reframe and ever after repeat to your similarly embarrassed friend as,
"That's going to be the first line in your obituary."