Plugging in powered speakers can shut Pi down

[zwbrbr]

(Putting this here by request from Ed, though it's really a PCB bug)

The tip of a 3.5mm TRS jack from a set of powered speakers can have a very different AC/DC voltage on it compared to the Pi. This can cause the Pi to shut down when a 3.5mm jack is inserted. It's probably triggering the Pi self-power-off circuit, since the shutdown is orderly. It's not specific to the outer contact of the audio connector; touching the tip to the USB housing or indeed to the negative terminal of the barrel socket has the same effect. HDMI tends to visibly glitch on the monitor at the same time.

Workarounds:

1. Use only headphones; even ones with batteries are unlikely to be a problem.
2. Only plug/unplug audio while Canute is off.