Comment 12 for bug 1474225

there is thresholding in the sensors... which is exactly the cause of this "bug".

When shaking, you make the threshold switch to landscape and by putting it flat down on the desk you keep it that way, not allowing the threshold to be hit to switch back to landscape. You just don't see it, because you're using an app that forces locked to portrait (the dash for example). As soon as you allow unity to rotate (by launching the settings app) it will catch up with what the sensors state is => landscape.

Now, one way to get around the situation would be to disable the sensors while unity is locked to some orientation. This however seems quite tricky, as on one hand that would need to be synced with what sensors tell apps, but also apps might still want to know the physical orientation even though locked to some orientation.

IMO this is working as expected, even though I agree it can cause some oddities like unintentional shaking. But in the end it's really what you tell the device to do. As an analogy, you're sort of asking for a mechanism to distinguish accidental display taps from intentional ones.