Old motherboards/circuit boards
...and anything else I've shot at
The only thing I don't pack out is broken glass, because that will grind back down into normal silicon dioxide sand and become part of the soil again fairly quickly.
However, a lot of people (almost everyone, it seems) don't pack their trash out. I've seen tons of shotgun shells, plastic soda bottles, ammo boxes, paper targets, bowling pins, propane tanks, even the occasional whole dishwasher or fridge. All dumped right out in the open. Not even an attempt to clean it up.
Well guess what, asshats? When you dump your trash in the wilderness, you ruin that wilderness for everyone. You're going to get the plinking spots in Left Hand closed down, and you damn well deserve it for being such inconsiderate, environment trashing pigs. This is a tragedy of the commons situation, and the consequences are inevitable. They might as well close the place down now.