Same mistake as many hunters make: not being able or willing to sit still. Movement really stands out and will spook a wild animal no matter if you are downwind or how well camouflaged you are. Squirming around to get comfy, twisting around to get a drink, shooing away flies, moving hands or putting binoculars up and lowering them repeatedly will get you busted.
A friend and I sat and scoped out a herd of antelope that were tight up against the hill we were sitting on because a hunter about a mile away was moving around and trying to "sneak up" on the little buck in the group. Before we knew it, they were all around us, looking back at him and generally staying at a safe distance from the perceived threat. We were both right there, but neither of us moved much. They moved right around us and went around the hill. I could have tossed a rock at the buck when he went past, but we sat still.