A little bit of well-needed silliness.
For all his clipped, nearly Victorian diction, his posh suits, his charmingly eloquent hand gestures, Sherlock Holmes is surprisingly vulgar and boorish within the confines of his own home.
John puts up with a lot, he really does. The detective will often continue a conversation from the toilet, door nudged open just far enough so that John will have to pointedly look in another direction. He'll stir tea - both his own and John's - with a finger that's covered in who-knows-what from some ungodly experiment. He fusses absentmindedly with scabs, which drives the doctor in John absolutely up a wall. He did have to put his foot down the time Sherlock started idly picking at stitches on John's scalp - ones he had earned chasing after the bloody git.
But this? This was just too much. Sherlock had been perched too far into John's personal space, as usual, prattling on about some absurd theory or some case or other. John had stopped listening about twenty minutes ago, unable to muster up the energy involved in pretending to care.
And then suddenly, without turning his head or missing a beat, Sherlock had opened that lush, gorgeous mouth of his and let loose, right in the vicinity of John's face and without so much as an apology, a great, emphatic, reverberating belch.