John and Lestrade recognise each other during ASiP. Years ago, when Lestrade was a young man prone to getting absolutely smashed every weekend (early twenties?), he had a one night stand with John. The catch? John was a teenager at the time, and underage. It was mutually consensual, as much as a teen with raging hormones and a guy completely off his face could be, but it's still illegal. Lestrade didn't realise this until the next morning, when he found a blue-eyed, blonde and obviously still underage boy in his bed.

They weren't in love. They weren't dating. It was an experiment for John, and an accident for Lestrade. I want to see how they react to each other years later.

Bonus if you touch on Lestrade's occupation as a policeman with this in his past.

Double bonus if you make this Johnlock, and Sherlock is aware of their one night stand. He accepts it...how long it takes him to, and what he does in the meantime, that's a different story.