We had this happen recently. There was supposed to be a climactic battle scene, which a companion ended with a lucky strike, matched by the opponent botching his defence (IIRC).
Kinda took the wind out of the scene, but since so much of Ars Magica is about dealing with the consequenses, I expect it'll work even better in the end!
Oh goodness yes. The guy who rolls an average of 18-20 on a stress die 
Not because of cheating, but pure luck. 
I ran an adventure where a demon was suppose to slow the magi down but an amazing roll ( Marko, I'm looking at you) carried the magi through. Roll with the punches and let the players bask in the glory.
Luck always turns and players who rely upon luck to solve their problems eventually run out of it. Of course, that can be fun, too! 
I remember when I was the lucky one. Rolled well. Seemingly all the time. Hardly any bad rolls. It was great. And then? I performed MATH! and SCIENCE! upon the dice. Turns out they were unfair dice. 