So I’m coming back from an errand early one morning, and I turn onto the farm road where I live. I live out in the country, you see, and the farm road is a long, straight road that rolls over hills and creeks between fences, woods, and pastures. As I’m driving down the farm road, I think to myself, Although it’s technically day time, on cloudy days, deer are often active later than normal. I remember a particular stretch of the road just before the creek, where a cow pasture faces woods and where I’ve seen deer before. Then, as the relative elevation of the road changes in relation to the pasture, I see a single deer in the middle of the pasture against the backdrop of the hills beyond, and I slow my truck to ten miles an hour. Where are your buddies? I ask him, because does and fawns travel together in family groups.
Two deer dart across the road, and two others turn from the fence they were about to hop and retreat into the pasture. If I’d been traveling normal speed, I very well might have hit one of them.
I know the general behavior of deer; I know the lay of the land and the geography of deer crossings; and I just might have seen the deer in my peripheral vision, outside my focus but enough to trigger additional caution until I did see it consciously. That’s how I knew the deer were there.
And that’s how I found that bug. I know the general behavior of computers, applications, and interfaces; I know something of the domain or problem this program is trying to solve; and I have wide peripheral vision when testing, the ability to see things wrong in the corner of my eye and retry my actions with focus on the problem area.