Once, some time ago, I was tasked with travelling to a Boeing lab in Seattle. Our new shiny piece of aircraft equipment was failing to start a dataload (a mechanism by which the software loaded on aircraft equipment can be updated, without having to take the kit off the aircraft).
We had spent some time previously on this part of the software, and were fairly certain that we were following the protocol correctly.
This protocol (as had been given to us) had quite strict timing for our equipment to respond with; just a few milliseconds (ok, so this was some time ago).
We had to use an expensive (5 figure $) protocol analyser to show that, yes, our equipment was following the protocol correctly.
It turned out that the aircraft wide, large corporation, provided kit that was the control system for dataload had failed to meet their side of the protocol. Our equipment responses were being ignored, so our system just assumed the process had timed out.
Score 1 (one) for byte-level checking of message data.
There followed the amusing telephone conference call with their technical team, of the form: Q: so, are you meeting the timing requirements for this protocol? A: well, I thought we were, but, err, um, we'll have to go and check..
<sound of silence as they mute their side of the call> ...
A: um, yeah, um, we'll have to call you back..