Wind. Every outdoorsman is familiar with it, from little gusts that cool a sweaty brow to steady winds that blow all day long. And they know that game animals and fish react to winds in various ways. Ask even a 14-year-old boy or girl deer hunter about wind and they'll say "Sure, deer have a great sense of smell, so you have to be careful about wind." But most do little about it when hunting time rolls around, nor do their elders.
I remember going on a series of deer drives one time with a bunch of friends, and listening while the hunt captain set up a drive through a big brush area, deciding where to put drivers and standers. As the standers hurried off, I mentioned that the wind was going to be blowing from the standers to the drivers, instead of vice versa, and the captain said "You're right, but it probably won't make any difference." We put out a small buck and three does on that drive, animals that knew perfectly well where the standers were waiting and heard the drivers coming. They went out one side, and nobody got a shot.