This happens to me every once in a while. Usually I swipe an email away completely on accident (maybe I was trying to swipe a different one). But I didn’t even see which email exactly I swiped away my mistake. How do you get it back?

The trick, and thanks for Matthew R. Lee for documenting this, is that from within the Gmail interface, search for in:archve. The search results for that will be all archived emails, ordered by when they were archived which is the clutch thing. The email(s) you just accidently archived should be right up at the top for you to move back to the inbox.