Not to the best of my knowledge - no, and as far as my parents know, this hasn't happened in the time they've been teaching.
Of course the SQA is far from perfect, though they got a deserved kicking up the bum back in 2000 when they got everyone's results late/wrong/both. I think since then, they're terrified to muck anything up, so everything is done on their tip toes.
I can't remember the SQA ever having to deal with a leaked paper - this of course doesn't mean to say it hasn't happened - who knows what has gone on that we're not aware of - but as far as I'm sure, there's not been a problem of this sort.
A few others though - just ask anyone who was due to receive their results four years ago!