I also had this problem—with the same stack—but the issue was quite subtle. It was due to an upgrade of the Money gem that changed the way "cents" were handled

I also had this problem—with the same stack—but the issue was quite subtle. It was due to an upgrade of the Money gem that changed the way "cents" were handled. In our case we had test coverage, including integration tests with VCR recordings to the payment gateway. But the problem was that the bug only affect Japanese Yen, and we did not cover every single currency.