Sometimes, when you place a Note Block from the Creative inventory, it isn't always at the lowest pitch (dark green), and may be set as high as red notes. This only happens when starting to tune it - I haven't tested it from redstone.

On the side note, I did all this under the circumstances of a nearby 'two redstone torch clock' (see this page for info: link coming soon ) , so maybe that may be the cause?

Anyway, just be sure to look into it!