We are developing an application; it includes a library developed by another coder, this library communicates with server via multiple network connections, and this involves multiple threads working together. The server side code is quite complicated, and we don't have access to the source code.
Recently I've discovered a mandelbug making application crash sometimes. I could reproduce it once and got a stack trace, so I opened a bug report. The bug itself is easy to fix (uncaught web exception in one of background threads, that makes CLR terminate the program).
The problem is that developer is refusing to fix the bug, because "he is not convinced it exists". Unfortunately for me the boss is siding with him and says this bug cannot be fixed unless I make a "solid test case" to prove existence of the bug, and to make unit test verifying that it's gone. What is basically impossible due to a nature of the bug.