I have done a bit MIG in my time, but never made such a silly mistake as today. I just could not get a decent weld, changed the head of the torch, checked feed through, feed rates, gas etc.
Doh! Stupid me, I was holding the joint together with a strong magnet (Sea searcher recovery magnet) which was sucking the filler and hence arc all over the place!
Lesson learnt.
Bob