I use to work as an assistant in NYC. Every single photographer that had Norman packs always had them blow up all the time. The first time I turned off a Norman pack it blew up on me with sparks flying out the side. I never trusted them since.
I've had a few Speedos and Dyna-Lite pop but worked with those packs for such a long time that it was a rarity.
Do what DKT mentioned about turning off power before changing plugs no matter what pack you are using.