Oh you know those are just glitches right! It happens to me all the time and if you cant backstab then just fire your revolver, enforcer, ambassador, or whatever at your enemies. The revolvers are very strong and often many players who play as a spy don't use them. I can take out a heavy with all six shots from a enforcer it that easy. So give your friend that tip to from me. Im pretty sure it will come in handy against those pyros.