Another one:

"No, that can't possibly be a fault - there's thousands of these in the field, and nobody's complained; it's a Proven Product..."

I had this very problem only a couple of months back, interfacing a telemetry device to a vending machine (VM) DEX port.
I already had an implementation working, and the Client had a requirement to talk to a new machine; I hooked it up and the data looked fine, but always failed the CRC data integrity check.
After some detailed examination, I was convinced that the VM's CRC implementation was wrong; but the VM manufacturer said, "No, that can't possibly be wrong - there's thousands of these in the field, and nobody's complained..."
I, of course, used exactly the same argument: "my implementation works fine with X, Y, and Z other manufacturers' machines, so it must be right!"

The manufacturer then insisted, "It must be right - we tested it with this software from the European Vending Association"

Looking at this "test software," I found that it doesn't actually check the Data Integrity CRC at all!

It eventually transpired that they had misinterpreted the spec, and their implementation was, in fact, incorrect - despite all those machines already in the field without complaint!