Imagine: John getting jealous over your friendship with Sherlock.

You, John, and Sherlock had all been friends, although you had a slight crush on John. You and Sherlock had to go on a case by yourselves so John could interrogate the witnesses. When John met back up with you and Sherlock, Sherlock told John how you and him had to pretend to be in a relationship, to which John reacted furiously to.

"You...flirted with Sherlock Holmes?"