When you realise your favourite series was originally only meant to be 2 seasons long but it's 15 seasons long with multiple mini series and shorts, a 3/4 season spin off series and a movie
When your writing an original story and start making headcannons for your characters but then you remember that your the author, so you have the power to make it cannon
Day 7 of my fandoms out of context:
"My mum is weird, and collects seashells. Your dad levels cities and attacks innocent people. So....they've all got their quirks, you know?"