Err, I can think of only one thing that will make a 100% attack fizzle. There is a balance Debuff that gives you -45% accuracy.
Sometimes when I was too absorbed into something else other than the fight, I would not notice that buff on me either until something fails that normally doesnt.
If it happens when you are dueling, I believe that is a known problem. Otherwise, there should be no reason for a 100% spell to fizzle unless you have a -45% debuff on you.
I don't know - something similar happened to my brother. He was fighting in Marleybone and his attacks fizzeled every turn until he fled. He ended up with maximum amount of pips and so did his opponent. That is why it can't be the debuff! His opponent was fizzling too! He was in the battle for half an hour until he finally fled!
