The kind of bug where a reproduction attempt takes a week, not parallelizable due to HW constraints, and logging instrumentation makes it go away or fail differently.
The hardest one I've debugged took a few months to reproduce, and would only show up on hardware that only one person on the team had.
One of the interesting things about working on a very mature product is that bugs tend to be very rare, but those rare ones which do appear are also extremely difficult to debug. The 2-hour, 2-day, and 2-week bugs have long been debugged out already.