I think the worst case I encountered was something like two years from first customer report to even fully confirming the bug, followed by about a month of increasingly detailed investigations, a robot, and an osciliscope.
The initial description? "Touchscreen sometimes misses button presses".
I'm no Raymond Chen, but sometimes I wish I'd kept notes on interesting bugs that I could take with me when I moved jobs. I've often been the go-to guy for weird shit that is happening that nobody else understands and requires cross-disciplinary insight.
Other favourites include "Microsoft Structured Exception Handling sometimes doesn't catch segfaults", and "any two of these network devices work together but all three combined freak out".
The initial description? "Touchscreen sometimes misses button presses".