I'm working on my 3 and 4th quilt. The first one was years ago, loaded with mistakes. I didn't even bother trying again until about 5 years ago, when I took a class with a great teacher. I made a rail fence quilt. It wasn't until I was completely finished quilting it that I noticed that I had put the two halves together wrong! They are totally opposite each other! Oh well! On my next quilt - the king sized trip around the world I'm working on - because the squares are much larger than the pattern I used, the center didn't come out like the pattern. My dh doesn't care - he likes it as it is, and since it's for our bed, it's fine with me also.
This next quilt that I'm working on? It's a disappearing 9-patch. I just finished cutting out the squares yesterday. I think due to the nature of the pattern, mistakes won't show so much :)
I doubt very seriously that anyone will be asking me to quilt for money any time soon, so I certainly don't stress over glaring errors :D