Some people using a Mac may have bad behavior with Keyboard.println eg. in the KeyboardReprogram example.
The problems stem from the Mac's not seeing the Arduino as a US keyboard. To facilitate experimentation, one needs to do two things: Set the default system keyboard to US. And also run the keyboard setup again, and execute a sketch that sends A SINGLE BACKQUOTE to the Mac when the setup assistant wants a key pressed. Don't ask ...
Someone smarter than me can flesh out this post; my sketch is on another Mac etc. and I will try to provide details some other time, but this may help someone.
Edmund