Considering that I'm a newbie to guitar playing it's no surprise that this happenes to me all the time. Most of the stuff I have been playing is classical music so of course it sounds great but I probably screw up 1 out of every 30 notes. My wife and friends all say wow that sounds great, meanwhile I'm cringing in my head every 20 seconds.
I know most people are being nice and saying that it sounds great but I hate it when it happens when I know I am playing like **** at the time.