My favorite is when someone looks at you through their riflescope.
Opening day 2009, I've already been in the woods a few hours. Lots of chanterelles, fresh deer droppings, but no sign of elk. 50 yards from coming out of the woods, I can hear a road hunter tromping the gravel. I hear him get excited, so I start yelling "man here!" at the top of my lungs. I made the mistake of thinking he'd hear me - should have just discharged my rifle into the ground.
While I'm still yelling and walking towards the road, I watch him step into the game trail, fumble with his rifle, pull up and draw a bead on me. Glad he didn't squeeze off a shot. When I reached him, I think he was riding more adrenaline than I was.
Edit to add:
I have watched them start looking in the direction I am facing, check it out, and if acceptable to them they then climb the hill to approximate the view from my glassing point.
Betcha those pukes have no idea what a sonic crack from a magnum rifle sounds like. A buddy I hunted with in the '80s would have no hesitation to shoot over their heads in their direction to both scare them and hope they would come confront him so he could deliver a good beat down. On more than one occasion, I witnessed him deliver a lesson in common sense.