Sherlock
Day 20 - Favourite canon reference/in joke
There are so many - that's part of why I love this show so much, they've done it incredibly well and it is absolutely littered with references. Oddly though, my favourite has to be "three patch problems", because I didn't read the originals until after I'd seen the show (I know, I am ashamed - but now in love with it all the more) and when I came across "three pipe problems" my Mum asked me what on Earth I was grinning so much about.
Harry Potter
Day 20 - Favourite canon reference/in joke
...okay, this doesn't technically apply, so I'm going to add in real life to the end. Because I am sad enough that there are a huge number of them.
There's getting bored in lessons, and my whole class (okay, it's a small class...but still) having "duels" using our pens as wands. On one occasion there was an argument about whether or not someone could duck Avada Kedavra and I was turned to as the Ultimate Harry Potter Authority...this is close. It was very funny.
But then, there's the fact that we actually have a DA at my school: Dumbledore's Dave's Army. My old Head of Year: he was replaced by Umbridge a woman none of us like, so we started a petition to get him back. Pinned to the notice board in our Common Room. Right outside her office. It was removed. Okay. Maybe it wasn't the best plan. But I liked it.