I've doubled my 500NE and it caused me to walk backwards a couple of steps. Apart from developing an attitude not to do that again, I was otherwise unharmed.

A friend of mine doubled it repeatedly for 6 or 8 shots in a row (ie 3 or 4 times) - he's got short fat fingers, with part of one missing, and he somehow got both triggers at the same time. Didn't scare him off. I was standing right next to him spotting for him and noticed him disappear completely from my peripheral vision every time it happened.