Happens a lot more often then you might think. Just give a thought to how the pad of a dogs foot is made. The pad slides the saftey off, and the claws hook and pull the trigger. The way dogs walk, its almost more likely that it will happen (should the dog step on the trigger area) than not.

Whenever I set my gun down, I try and lay it so that the safety is down, meaning that if the dog did step on it, he could not push the safety off. More often than not (and always in the boat) I'll just put it back in the gun box, even if just for a second.