The bug ticks most of the boxes for a tricky bug:
* Non-deterministic
* Enormous haystack
* Unexpected "1+1=3"-type error with a cause outside of the code itself
Like sure it would have been slower to debug if it took 30 hours of to reproduce, and harder he had to be going down the Niagara falls in a barrel while debugging it, but I'm not quite sure those things quite count.
I had a similar category of bug I was struggling with the other year[1] that was related to a faulty optimization in the GraalVM JVM leading to bizarre behavior in very rare circumstances. If I'd been sitting next to the right JVM engineers over at Oracle I'm sure we'd figured it out in days and not the weeks it took me.