This is a corollary of the old expression, "Everyone thinks they are an above-average driver, so half of everyone is wrong."

I never have met a girl who thought that she gave a bad blowjob. Whether it's just mild sexual innuendo like...

"You suck."
"Yeah, and I do it real well. :) "

...or the women who expectantly look up at you after finishing the job expecting you to tell them they're the second coming of Christ or something.

I have this nasty habit of telling the truth a lot, and one time after I was the recipient of said blowjob my then-girlfriend (we had just started dating) asked me how it was. My reply of "It was alright...but please, less teeth next time?" was enough to send her storm out of the room crying...