Releases from 1.5.6 to 1.5.9 have a bug where keys sometimes get stuck in the on-screen keyboard. You have to quit the App to clear the keys.
This is already fixed for the next release. In the meantime, the workaround is to tap the octave selector rather tan drag it, so that the C key is always to the left.
Posted in: Bugs Fixed in 1.5 to 1.6