Sometimes you just have to dive right into the guts
Long ago, we were running a mix of VAX/VMS & RSX11 over DECNet, but the PDP11 kit was using 3rd party DECNet software. So one day we added some new nodes & started getting communication problems on the 11s. I passed the problem to the software vendors but they didn't seem to have a clue so I started looking myself.
The DECNet protocol was a bit of an eye opener - my first introduction to networking protocols - but I worked out what the various modules in this software did. I managed to get hold of a PDP11 disassembler & started looking at the module code. This was in the days before hex so the constants all printed out in octal. And guess what stood out - an octal 256 constant! WTF? So I changed it to octal 400 as it should have been, reassembled the code and it worked perfectly.
The constant dimensioned the node table (must have been DECNet phase III) so it overflowed when we added nodes above 173 (octal 255). I also discovered how the software bypassed the licence check for testing purposes, which was handy for quickly adding new nodes before we got a new licensed copy...