I used to be a programmer. I hate these kind of bugs. HAAAAAATE hatey hate hate. Give me a bug that you can demonstrate reliably and on command, and I'll track it down and make it a very dead bug in a very short amount of time. But a bug that just shows up occasionally, or once, for no discernible reason, and just whenever in the process if damn well feels like? HAAAAAATE. Especially if it just happens for the user, and all he can do is try his best to explain what he was doing and what, exactly the bug was? So much hate. Nothing against end users, but they aren't programmers, and they usually have no idea what exactly I am looking for or how even the smallest detail left out could hide the bug.
I'd look, of course. And occasionally the bug would be found and fixed. Most got marked CNR, could not reproduce after a few hours, and logged for the next poor bastard who ran across it.
I have much sympathy for Minecraft modders. Some of these mods are huge, and all are fairly complicated.