If you've got a good idea of what the customer is experiencing (logs/descriptions/whatever you can get for info), using the source code work backwards from what

If you've got a good idea of what the customer is experiencing (logs/descriptions/whatever you can get for info), using the source code work backwards from whatever code could generate the state the customer described to determine how it got there. At some point you'll probably come across some bit that looks weird and that's quite possibly the problem. It can be a bit of work, and it presupposes a certain understanding of the system in question, but one can absolutely do this. It's basically my day job.