A lifetime ago I was writing code for airline data processing. The specs are very clear about what the valid representation of every field was (less so about wh

A lifetime ago I was writing code for airline data processing. The specs are very clear about what the valid representation of every field was (less so about what they meant, but...). So we generated our parser to fail if field ORG/1457 (made up) was not numeric max 8 digits. Or missing where mandatory. Even if we never touched the data in that field. Turns out that no-one else used the spec that way. No two were the same, so we had to basically implement two layers of parsing. One to put the data in a common parse tree, and the other to per-sending-mainframe interpret the data as how the sender had implemented. We assumed that the mainframe would never send illformed data, and indeed that-could-never-happen. But they differed in what they thought was well formed.