Every once in a while when Sherlock's bored, John starts asking him about hidden talents/skills. Sherlock always claims the most outrageous things ("Oh, I went undercover as a lion tamer, nothing to it really!" or "Yes, I spent a month posing as a masseuse in a nudist resort" or "I actually did an extensive study of septic tank cleaners' daily routines") and John can never tell when he's bluffing. So eventually he dares Sherlock to prove it.

Could be smut (lots of wonderful things Sherlock could be an expert in!) or just generally crack, whether or not John catches Sherlock in a lie :-)