John has tolerated the loud bangs and awful rotten-egg stenches that diffuse slowly and nastily throughout the flat.
He has turned a blind eye to the fingernails floating merrily in every bottle of the six-pack of Stella he had bought only that morning.
Hell, he's even put up with Sherlock experimenting on him, sitting still in resignation whilst the other man paws through his hair with occasional exclamations of 'aha' or 'thought so'.
What John will absolutely not, a hundred percent not tolerate, Sherlock is the gently decomposing corpse that he finds waiting for him in his bed one horrible evening. The absolute lack of respect and propriety is what finally makes him snap, I mean my god, it's like something out of the bloody Godfather, what the hell were you thinking? Don't you have any concept of personal boundaries?
When Sherlock replies in the negative John grabs the corpse and dumps it onto Sherlock's bed, rips all the sheets from his own bed and has the bill from the most expensive dry-cleaners in London charged to Sherlock's bank account.
And then two days later he switches a select few of the detective's chemicals around, so that when Sherlock next goes to mix a simple pickling solution there is a sudden blast and his eyebrows are singed off.