I used to do architectural visualization. This firm hired me as a freelancer to create multiple illustrations of new condo buildings, including exterior shots, interior shots, floor plans, etc. Lot of work laid out in a well-defined contract, with costs agreed upon. Half the fees needed to be paid upfront, and the other half at the very end (after final approval). Everything went relatively well until the final payment was due:
Client: I cannot pay you the final amount. You see, in architecture, costs are calculated per door, and with the money that I gave you in the beginning, that more than enough covers for the work.
Obviously, I had created tens of rendered images of the same building, which they considered to be the same doors, so the same work, thus refusing to pay for the actual amount of work that had been done.
After a bit of back and forth, the client kept acting like a condescending twat and I had to give up most of the amount. Lesson learned.