Thanks for reporting.
After some effort I was able to reproduce this: if you switch to the database just when the tab bar is sliding down in expanded calculator mode, this occurs. Will be fixed in next update.
As there's been a bit of confusion with expanding/shrinking of the keys and device shake was probably never a good idea to begin with, I'm planning to have the calculator always in expanded mode, with the tab bar always sliding down after a short delay and never showing in semi-transparent mode when in non-calculator modes. This change is foreseen for v1.4.
By the way, you don't have to shut down your device. If an app misbehaves, double-click the home button and kill the app in question by holding it until it wiggles, and then tapping the delete symbol.