It's a Lunar Phase Dependency.
One debugger I used actually had an option to display the phase of the moon :). And actually that debugger (which was otherwise quite good) had a bug that made the execution misbehave when stepping over some instructions under specific conditions. As you might guess, I was using the debugger trying to find a bug, so I ended up with what I thought was a single bug that was behaving differently depending on how close I was trying to observe it. Lunar Phase Dependency alright.