It’s like everyone is at Comic-Con to celebrate the things they like, and if those things happen to overlap with other people who are at the same convention, all the better.
That being said, there are plenty even the most casual of fans would immediately recognize.
I write about movies and television for a living and have for over a decade, and I still come across dozens of costumes every single year that I can’t place.
Sometimes it’s because a particularly creative soul has mashed two different characters together to form some kind of hybrid and sometimes it’s because the costume is from an obscure tv show or movie I’ve never even heard of.