That's actually quite easy to reproduce.
I just created a new instance of the widget and typed in "×" (the multiplication symbol), "5", and "×" (the multiplication symbol) again. Obviously any number and any operation symbol works, but I have a natural affinity toward five for some reason and always use it in tests such as this, and I chose multiplication for no reason.
There are most likely countless ways to do this.
I first thought of overflowing it, but it froze instead of displaying an error message.