Event - I was hired by a couple to paint their living room. They gave me the paint color and left me to work. Turns out the color was wrong by a few shades. They had written down the color for me and I misread it. I told them I did it correctly and they screwed up, then had the paint store guy (who I do a lot of business with) back me up. They eventually believed it was their fault.
Apology - I ended up repainting it for half the cost because I told them I felt bad about the situation. But I should have just done if for free and taken the financial hit myself. I did end up adding some crown molding later for free.