You never see it coming

The problem with the dryer plugs is they aren't designed for daily unplugging. They're really meant to plug in the dryer once then not be touched again for many years. When you use them on a welder, they get plugged/unplugged many more times....and each time is an opportunity for a zap. I've also noticed the female receptacle in the wall doesn't hold up well to lots of use, either.