These things do happen from time to time - No explanation just that they do happen.
I was reading an article the other day about how CPU's are not as predictable as we like to think.
See here http://www.newscientist.com/article.ns? ... 725074.600
Anyway, it's probably best to forget about it as it may never ever happen again
Glad you figured it out though
Cheers,
Andy