From Wikipedia: "In anatomy, heterochromia refers to a difference in coloration, usually of the iris..." What do you think when you hear this word? Here's what I think:
DESU DESU DESU DESU DESU DESU DESU DESU DESU DESU DESU.

If you couldn't understand from the definition, I think our Suseiseki here explains it pretty well. Basically, two different colored eyes, or desu/anime eyes. It's probably one of the most iconic things to hit anime since cat ears and gigantic eyes. However, it isn't limited to anime. People such as David Bowie (bamf) and Tim McIlrath, the lead singer of Rise Against, both have it. Overall, if I had to choose a defect to have, this would probably be it.