I've not done a lot of commissioning lately, but in my past I've had situations where the artist I've commissioned comes to me with the finished product and lets me know that for whatever reason, the amount we agreed to/I paid them was not enough.
For instance, around four summer's ago I ran into an artist who made some amazing Silent Hill statues. I was horribly disappointed that McFarlane (or someone else) had not made any figures based on the movie, terrible as it was, so I commissioned this artist for one.
He quoted some prices to me, and I accepted. He was an absolute joy to work with, kept me updated constantly, was kind enough to answer my questions about his sculpting techniques, etc. However, when he presented the final product to me, he told me that he had been wrong in his estimation and that he had only broken even on the project.
That made me feel pretty bad, because this statue is just gorgeous, but at the time I was a college student who only worked during Christmas and the summer. I wasn't exactly rolling in money, and my parents still poke fun at me for dropping so much money on it in the first place.
I thanked him and praised his work profusely, but I did not send him any extra money (although looking back on it I wish I had, I'm still very much deriving a lot of enjoyment from his art). He deserved a tip, I think.
Later on, I commissioned a very famous DeviantArt artist to make me an icon. She offered still and animated options, and I opted for non-animated since I was a bit low on expendable funds.
Apparently she had gotten my order confused, because when she was done she presented me with an animated icon. I could have read her delivery note wrong, but I got the impression she would have liked me to pay her the difference for her mistake.
So, my question is, how do you feel about those sorts of situations? Would you feel responsible for reimbursing the artist for their mistake, or would expect them to honor your first contract? Would the situation change depending on your current financial situation? If you were the artist, would you expect the commissioner to pay you extra after the fact or resent them for not doing so?
All in all it's kind of an awkward situation, so I wanted to know how you would handle it.