And this is why your agreement has terms like fees for modifications to the original requirements

Finalise PCB
Has had the customer sit there looking over the cardboard mockup of the PCB, try it in there enclosure, etc to make sure everything is exactly to there liking (print flat renders on some card stock and use a vynyl cutter to cut the mounting holes and shape) (Try and get a CAD model of a case out of some of these people!)
Order PCBA services
2 days pass, just long enough for the boards to be manufactured, and the parts to be ordered
Customer has a dream about changing something in the middle of the night, and it absolutely needs to be this way now.
And the part I love, Emailing the quote for modification and re fabrication.