I used to get this from my 2013 MBP when I used it without the extension cord (which at least in Australia is earthed). For some reason the extensionless plug d

I used to get this from my 2013 MBP when I used it without the extension cord (which at least in Australia is earthed). For some reason the extensionless plug didn't have a connection to the power supply's earth. It was a bit disconcerting, but never caused me an actual problem.