Not an encroachment incident but one story of my dad's that was a favorite was when my folks and several other sets of friends were camping in their RV's (1 Class C, 1 Class A and a couple TT's) in an "unofficial" campground. They were all set up including screen tents, grills, chairs, tables, etc. covering a rather large area up from the shore of the local Corp lake. Around 1am or so, long after they'd gone to bed, a group of younger folks showed up in an area next to them and proceeded to pitch a tent and get drunk(er) and rowdy with loud music, yelling, screaming and laughing. Requests to have them quite down fell on deaf ears and belligerent attitudes.
One of the guys camping in the group was a hunter/trapper/fur buyer and happened to have a cassette of mountain lions mating sounds in his pick-up. So, about 6am when he woke up he got his boombox from the camper, carried it closer to the tent campers, turned it up as loud as it would go and played that tape of the worst cat screams you can imagine. My dad said he'd never seen anyone tear down camp as fast as those kids did!