I used to work as a projectionist in a movie theater that showed primarily classic films. This involved significantly more work than is required with modern films because of the need to switch between reels. In a nutshell, films used to be distributed in a stack of canisters each containing about 20 minutes of film, called a reel. Showing a film consisting of reels requires two projectors, and the projectionist has to load up each reel on the currently unused projector and wait for a cue to start up that next reel, which was loaded up such that the actual picture would begin exactly 10 seconds after the projector was started. Then, a second cue tells them exactly when to swap between the two projectors (in my case there was a floor pedal that would instantaneously close one projector and open the other). If done correctly the movie would appear to seemlessly transition from one reel to the next and audience would never be the wiser.

So what are the cues that the projectionist uses? In every movie you have ever seen (chances are) a small black dot appears in the upper-right hand corner of the film at 10 seconds remaining, and 1/4 second remaining. Now that I know it's there, I can't "unsee" it, so I notice it in pretty much every film I see in the theaters. Despite the fact that most films released nowadays are made of stronger materials and put together into one huge reel, making the projectionist job significantly easier and the dots superfluous, they are still there.

Even if you do know they are there, they can be very hard to notice unless you have spent a fair amount of time looking for them. To me, the most interesting part about the dots is that I've yet to meet someone who when I tell them about the phenomenon for the first time says "oh yeah, I always wondered what those were for!" Most people say, "No way, I would have noticed that" and are somewhat incredulous that this could have been happening in every film without them consciously processing it.

It just goes to show how powerful the brain's capacity to filter and make sense of the world is. Since those dots appear to have no meaning in terms of the film content, they are dismissed at some lower level of processing as perhaps some dirt on the film or a one-time anomaly to be corrected for. The brain is compensating for the eye's blind spot all the time, so throwing out a little black dot every so often is a piece of cake.

The black dot problem, as it relates to debugging, is the problem of failing to perceive useful information because it is grouped in with a larger, consistent data set.   For example, a rare but recurring error buried in hundreds of megabytes of uniform log files could be the black dot that you are missing when searching for the cause of a system crash.  Sometimes, simply being told that the "black dot" is there is enough to allow you to find it.  Other times, it's just a matter of stepping back and repeating to yourself that the information is there and trying to force those subconcious filters off for a while to really see the data in front of you. However it comes about, when you finally notice the dot, you will wonder how you could have missed it for so long.