It's not actually meaningless. It means "I did test this and it did work under certain conditions." So maybe if you can determine what conditions are different on the customer's machine that'll give you a clue as to what happened.

The most obscure bug that I ever created ended up being something that would work just fine on any machine that had at any point had Visual Studio 2013 installed on it, even if it had since had it uninstalled (it left behind the library that my code change had introduced a hidden dependency on). It would only fail on a machine that had never had Visual Studio 2013 installed. This was quite a few years back so the computers we had throughout the company mostly had had 2013 installed at some point, only brand new ones that hadn't been used for much would crash when it happened to touch my code. That was a fun one to figure out and the list of "works on this machine" vs. "doesn't work on that machine" was useful.