I've never had a perfect quilt yet, some have been close... but I've never had to make a deliberate mistake plenty happen on their own. Sometimes the mistakes get left in, sometimes they get fixed.
I did a scrappy Storm at Sea and it had been quilted and hanging in a guild show before I noticed one of the fabrics was wrong side out. Now, we have two sides to fabric the one we intend to use and the one we did use -- I planned on the other side. I figured I had handled each block about 24 times during just the construction part and never caught it, quilted it all over and didn't catch it, did the binding by hand.
Short form: had I noticed this one, I would have fixed this one. But no one else has noticed it and the gifted owners never mentioned it either. I feel ok about not fixing it... working with scrappy style I will put in special fabrics with my own meanings for the recipient. Sometimes I put them in without mention, sometimes I let the people know. I think nowadays (20 years after that Storm at Sea) I would say that "Btw there is one piece in there that's backwards -- gives you something to look for when you're sick in bed".