What's worse is when you have to do worldbuilding in fanfiction! Crossover Harry Potter with Marvel, Star Trek, Doctor Who and so on, and you suddenly find yourself having to figure out where Potterverse magic comes from! Because the likes of Dr Doom, Charles Xavier, Mr Spock and Commander Data are not going to quit asking and looking until they figure it out! Also, The Doctor will already know anyway!

Then you have to explain why the Sorceror Supreme (Dr Strange as per comic canon) is muggle-born!