I've been working with Swift since it was launched in the Xcode 6 betas. It has been challenging. Working through all of the betas and going with Swift was probably an insane choice given that Swift was in such a state of evolution. Every beta would cause a sea of red flags. I'd dread having to level-up when a new beta was dropped, but I figured, might as well get it over with than wait until the GM hits and have things really be in a terrible state! My vote for the "best" error that I received during development was in some computational matrix code that calculates flight paths: "Expression was too complex to be solved in reasonable time." Of course, I read that as, "The math is making the room spin up in here."

However, I dig the language, and things have stabilized significantly now.