Motorcyclists have a saying: you go where you look. If an animal runs out in front of you, or you there's a patch of sand in the middle of the corner, or a Corvette is coming the other way, your first response has to be to look elsewhere. If not, you'll almost certainly hit whatever it is that you didn't want to hit.
Another name for this phenomena is target fixation, and that name was driven home to me quite literally — and painfully — in a paintball game many years ago. I was slowly and carefully positioning myself to shoot one of the other players, when all of a sudden I felt a paintball hit the middle of my back. I was so fixated on my target that I stopped paying attention to what was around me.
I suspect that target fixation was an enormous help to our hunter-gatherer ancestors stalking their dinner. They would only get one chance to bring down their quarry, and didn't have the benefit of high-powered rifles and telescopic sights. To a modern human, surrounded by opportunities to fixate on the wrong thing, it's not so great.
Physical dangers are one thing, but we're also faced with intellectual dangers. If focus too closely on the scary thing that's right in front of you, you'll ignore all the pitfalls that lie just beyond. This is a particular concern for software developers, who may adopt and implement a particular design without taking the time to think of the ways that it can fail — or of alternative designs that are simpler and more robust.
For example, you might implement a web application that requires shared state, and become so fixated on transactional access to that state that you don't think about contention … until you start running at scale, and discover the delays that synchronization introduces. If you weren't fixated on concurrent access, you might have thought of better ways to share the state without the need for transactions.
So, how to avoid becoming fixated? In the physical world, where fixation has potentially deadly consequences, training programs focus on prevention via ritual. For motorcyclists, the ritual is “SEE”: search, evaluate, execute. For pilots, there are many rituals, but one that was burned into my brain is aviate, navigate, communicate.
For software development, I think that a preemptive “five whys” exercise is a useful way to avoid design fixation. This exercise is usually used after a problem occurs, to identify the root cause of the problem and potential solutions: you keep asking “why did this happen” until there are no more answers. Recast as a pre-emptive exercise, it is meant to challenge — and ultimately validate — the assumptions that underly your design.
Returning to the concurrency example, the first question might be “why do I want to prevent concurrent access?” One possible answer is ”this is inventory data, and we don't want two customers to buy the last item.” That could lead to several other questions, such as “do I need to use a database transaction?” and “do I need to make that guarantee at this point in the process?”
The chief danger in this exercise is “analysis paralysis,” which is itself a form of target fixation. To move forward, you must accept that you are making assumptions, and be comfortable that they're valid assumptions. If you fixate on the possibility that your assumptions are invalid, you'll never move.
You also need to recognize that, while target fixation is often dangerous, it can have a positive side: preventing you from paying attention to irrelevant details.
I had a real-world experience of this sort a few weeks ago, while riding my motorcycle on a twisting country road: I saw a pickup truck coming the other way and not keeping to his lane. With a closing speed in excess of 100 miles per hour there wasn't much time to make a decision, and not many good decisions to make. I could continue as I was going, assume that the driver would see me and be able to keep within his lane; if I was wrong in that assumption, my trip would be over. I could get on the brakes hard, now, but would come to a stop at the exact point where the pickup would leave his lane while exiting the corner.
My best option was to stop just past the apex of the corner, which would be where the pickup was most likely to be within his lane. I fixated on that spot, and let the muscle memory of 100,000+ miles balance the braking and turning forces necessary to get me there. I have no idea how close the truck came to hitting me; my riding partner said that it was an “oh shit” moment. But once I picked my destination, the pickup truck and everything around me simply disappeared.
Which leads me to think that there might be another name for the phenomena: “flow.”