goodroi - 10:04 am on Aug 9, 2013 (gmt 0)
I have come across times that Google did not follow robots.txt. It happens .001%. 99.999% of the time it is because someone else didn't do something right.
I know you say that robots.txt hasn't changed. Double-check it anyway.
Did you add Google+? Google+ will create a backdoor for Google to access pages regardless of robots.txt.
Is your hosting company reliable? It is rare but sometimes a cheap hosting company will not load up files correctly.
If you really don't want Google to access certain pages consider using htaccess.