Strange result of String.fromCharCode

You are right, it works perfectly well on a clean device.
It seems like passing a wrong argument (the '\x07' string) causes the error:

>String.fromCharCode('\x07')  // wrong argument, but no complaint
="\0FD"
>String.fromCharCode(7)     // correct argument, but extra chars
="\7FD"
>String.fromCharCode(8)    // correct argument (above 8 every value gives the correct result)
="\b"
>String.fromCharCode(7)    // correct argument below 8, always with extra chars
="\7FD"

(It was pretty hard to find out how to reproduce the effect.)