I write a story about Angels and Demons, and I draw all Demons/only the Demons with sharp teeth/fangs. People were surprised when I revealed one of the characters was actually a Demon although he was one of the few characters with fangs XD no one had noticed, so I had to point it out...
I also tend to joke a lot about having same face syndrome to cover up the fact that characters who look a lot alike are either the same person in disguise, or related....
Also, changing the eye colour of characters who are possessed. My readers assumed that one character constantly switching between blue or green eyes was a mistake, but it was an indication of when she's possessed or not.
Some of this stuff can be spoilers so not sure if I should share the link XD