Beyond the numberless people cutting switchbacks and creating fire-pits above treeline, a few of the worst behaviors I've seen:
My first hike getting back into backpacking in May '08 to Tiltill Meadow via Rancheria Falls in Yosemite. It was Memorial Day weekend. In the parking lot with me getting ready to head out was a group of 4 guys and 4 girls. This was several months before I discovered BPL, and I was no lightweight, but even I could look at the glass bottles of hard alcohol they were packing and see they were if for a painful hike. It started to rain not too long into the day. I made it to Rancheria, where I was spending the first night, long enough before them, that I ran into them on the trail while I was poking around the falls. It was about an hour before nightfall and they were asking the time. The decided they could make Tiltill that night. I ran into them the next day about 1.5 miles from Tiltill. It had rained all night and they looked miserable. I chatted with them about the conditions in the meadow. When I got there myself, and started scouting for campsites, I found what had obviously been theirs. They left what seemed to be all their trash in the fire pit, 4-5 bottles of vodka, plastic bottles of mixer, plastic wrapping; they had attempted to burn it, but, well, their wet-weather fire-building skills were as good as mine, which is to say, non-existent. My night spent there, I encountered another large group who hiked in. When I left early next morning, I saw they ditched the required bear canisters in favor of bear-hanging their bags all of 5' above the ground. If they hadn't thoughtfully hung their bags 20 yards from where I'd set up (100 yards from their site), I would've been happy to have seen their bags ripped to shreds by Yosemite bears.
I befriended a guy who was trying to hike every trail in Yosemite during my own loop hike of Sunrise, Merced Lake and Vogelsang High Sierra Camps. While we were hanging out at Merced, a teenager in a guided group attempted to pull down a seeming dead but stil-standing tree for firewood. The guy, Bill, told him to stop and explained the rule was dead-and-DOWN. He stopped, but his group's camp was right next to ours. How they planned on burning a 6' long, 8" diameter log, I have no idea. But for the rest of the evening, they made a point of heading us off at the water pump whenever they saw us going to refill, perhaps because we made a point of strolling through past their camp with bundled firewood we procured by hiking a quarter mile back down the trail.
And, of course, anyone who hikes the trail up to Half Dome will encounter this sign:



