What if Giles (Rebellious teen/Long-suffering watcher of the vampire slayer) knew Crowley (Pissy cross-roads demon/King of Hell)?
There are so many possibilities and they all start here:
Obnoxious and dangerous teenage wannabe sorcerer "Ripper" is carrying on with his obnoxious and dangerous teenage activities probably putting his and others lives in danger (well...Ethan's) when he's interrupted by an arrogant and frankly very rude cross-roads demon.
They tend to disagree and neither of them take it very well. And they totally don't just get into a pissing contest because they're certainly not both insufferable idiots (wonderful idiots but idiots nonetheless).
This is the ripper-time prologue to a number of Giles and Crowley au's soon to be...uh...written.
Prompt from kink-meme:
Sherlock and/or Mycroft think they are the God of Sex. Their enormous ego dictates that no one can best a Holmes, especially not when it comes to the delicate art of lovemaking.
In reality, they are just really bad at it.
And every so often John and Lestrade meet up in a pub to grouse and grumble about having ended up with the worst lay in history.
Alternatively, just describe a really bad sex scene between John/Sherlock or Lestrade/Mycroft. Where the Holmes denies it being his fault and insists that he is fantastic.
...Basically Mycroft and Lestrade have really bad sex and Lestrade complains to John.