You can not trust the Calculator app in iOS 11

If you rely on the system Calculator app for your day-to-day living, please take note that you might not always get the correct calculation especially if you have quick finger.

There is a bug in Calculator app for iOS 11. Try tapping quickly “1+2+3” and you will not get the correct result.

This sounds scary indeed as your iPhone is basically a computer and it can’t get such basic calculation right.