The most expensive EDI failures I have seen were not mapping errors. The maps were fine. Nobody had asked how the document was actually used.
Before I map a single transaction now, I ask five questions. Who touches this document after it arrives? What breaks when it is late? Who gets the phone call when it is wrong? What does the current workaround look like? And who decided it should work this way?
The answers tell you more than the spec. They tell you where the real process lives, which is usually in someone's head and a spreadsheet.
This is the FDE discipline applied to integration: discovery before code. The questions are the product. A map built on good discovery survives. A map built on the spec alone gets rewritten in three months.
It also changes the conversation with the business. You stop being the person who moves documents and start being the person who understands the operation. That is a different career.
Five questions. Ten minutes each. It has saved me more rework than any tool I own.