What was the bug?
It was a nasty one. Any stat point bonuses granted from weapons or armor are displayed in the level-up screen, added to your base stats. When you spent stat points and confirmed, the bonus points from your items ALSO were permanently applied to your bases stats. Players quickly and accidentally became indestructible and extremely powerful, completely breaking the balance and fun (that one's objective, I suppose) of the game.
How did it happen?
I just had to slip one more feature/UI tweak in at the last second. A couple of days before I submitted to the App Store, I made a change to the level up window that added item stat bonuses to your base stats and displayed that stat in blue, for convenient display purposes. The bug spawned from the fact that I was saving this combined total to the player's base stat instead of saving the base stat minus the item bonus.
The real crime was adding in a feature before release and not thoroughly testing it. I made sure the change worked ("Awesome, the combined stat displays correctly!"), but I failed to regression test and see that the feature had repercussions that I didn't account for.
I guess that's the takeaway from this - don't assume that a minor change won't have a major impact on another part of your game, even if it does what you intend it to. I learned my lesson on this one, but it unfortunately came with the price of ruining the experience for my first players, which makes me sad.
Despite this hiccup, the feedback from players has been overwhelming. I'm well aware that the App Store is oversaturated with a sea of apps and getting noticed is beyond difficult. I would've been happy to see one happy player, so I'm ecstatic that so many people are playing and enjoying the game and can't wait to get more awesome features out the door and into the app.