In the US I’ll say littering. Some areas have available trash receptacles well-placed, but that’s few and far between, and some of those places don’t have good systems in place to collect that trash.

But even with that, I get pretty bent out of shape at the amount of littering in nature areas. Families tend to be the worst at this for some reason. Into the mountains to do some sledding, well see someone has left broken, busted plastic sleds and foodstuffs behind. A river? Sandals, swimsuits, broken chairs, foodstuffs. The Forrest for camping? Busted equipmemt, sleeping bags, foodstuffs.

Now, it less common in that not everyone is doing it, but more common in that I usually see at least one example of this at every nature site I go to.