I have an expired US student visa and an approved, but not stamped, US work visa (this is a totally legit situation). I am planning to travel from the US (where I currently live) to India for my visa interview for my new work visa.
Surprisingly, many European airports (Frankfurt, London) don't allow you to travel back to India in such a situation, without a transit visa. Does South Korea also have a similar policy? I am planning to transit via Incheon.
UPDATE: As I mentioned in the comments, the Korean embassy didn't really give me much guidance but I was able to make my trip successfully via Incheon.
Tl;dr If you have an expired US visa that you intend to renew, you can fly via South Korea without requiring a transit visa.