The 12th bar shouldn't be black. I don't know if it's a case of the graphic getting corrupted on some devices, or triggering some obscure iOS bug, but it seems to occur randomly to some people but not to others. On my own phone, the release version of the app shows the bug, but the latest development version doesn't. It's very repeatable but it doesn't make sense because the code and graphics are the same. There's also nothing different about the last bar compared to the other 11.
One of these days I'll get some time to finish an update, and with any luck it'll make this problem go away.