Before you can fix it, you must be able to break it (consistently).
(non-reproducible bugs ... Heisenbugs ... are extremely difficult to deal with)

If you can't find a bug where you're looking, you're looking in the wrong place.
(taking a break and resuming the debugging task later is generally a good idea)

It takes two people to find a subtle bug, but only one of them needs to know the program.
(the second person simply asks questions to challenge the debugger's assumptions)

(In fact, sometimes the second person doesn't have to do or say anything! The process of explaining the problem is often enough to trigger a Eureka event.)