The one that I have seen that was snappy was actually having a frustrated herding instinct, not remembering the difference between driving their handler and 'heeling'.
I saw a little girl with one in Obedience who nipped her ankles during heeling, so she wore thick socks till she got it stopped ;~D
My friend's dog was not anything like that though.
When I was little, we had a Blue Heeler mix...She used to chase us kids into a corner, then bark till my Mom came down to rescue us; I hope to find a dog with similar instincts.