Sherlock and John play out a (completely consensual) rape-fantasy. Unfortunately, Mycroft has cameras everywhere, and one catches the struggle before they go to the bedroom. This gives him the wrong idea, and he and a team of minions show up at the flat.

Bonus points if Sherlock is the 'attacker'.
If it's John as the aggressor, bonus points if sherlock's indignant when the misunderstanding comes out- "as if I'd need rescuing, Mycroft, really!"