Why would the tipping screen have a negative sign? Not that I haven't encountered service that I wished I could've tipped a negative amount for . . . .
It doesn't necessarily need to be a '-'. Find a value for the tip that overflows the variable type, possibly. I'm not suggesting that this is likely. Curiosity above what code is running under the covers is all.
Has anyone managed to try tipping a negative amount (to fill a security curiosity )?