I used to work in third-party logistics and a big project of mine was an automated file import process, so folks could send us their daily/hourly orders for processing and fulfillment. I'd say roughly 65-70% of the entire code base was error handling and figuring out when to kick out a file for human review and/or outright deny it and contact the customer.

The hardest ones to work with were the mom and pop shops who suddenly had some success on Amazon and came to us after fulfilling out of their garage for a year and a half. Try telling a semi-retired 60 year old electrician in the middle of Iowa that the file he sent is worthless because none of the product codes match what you have, especially when once he closes the file he doesn't have any idea where it is.