There might be many situations, but I was mostly talking about virtual vandalism. When you edit points, lines or routes in the wrong way, they get damaged. The volunteers guarding these need to fix them themselves, which can be a burden and they can get emotional already. You can imagine how that goes when more damage gets done where the fixing is left for those volunteers.

I don't know much about land-owners doing vandalism, often people try to communicate, sometimes this works out okay.