I am so glad you didn't ask "how many" mistakes I have made. LOL
I think my BIGGEST mistake was a few years ago at retreat. Decided to do a Minnesota Hot Dish pattern (easy), had all my fabric cut and even made a sample block. All set to do the 80 blocks. Sewing away, doing 40 blocks, chain piecing at a time. Was about the third round, ready for the fourth round. oh, %$*&)**($ All 40 of the third round is on the wrong side and will make a differences of the layout of the quilt ..... spend a lot of time unsewing...