I think the biggest 'douche' move that was done to me was:
I was a low level Empathy Defender, just starting out in The Hollows with a new character. A duo of higher leveled people appeared and offered to help me with missions to which I accepted. Did a couple of missions with them where I focused primarily on healing them while they did most of the fighting. After a bit they offered to power me a couple of levels so we changed rolls and I sidekicked to them. They hit me with the Kinetic movement stuff so that we could go out to the higher level area to kick some butt. Once out there we stood around and talked a little, about what I don't remember. But later I found out that they were just trying to stall for time for the Kinetic movement buff to wear off. As soon as it did they dropped team and took off, leaving me in the middle of an area with exceptionally higher level mobs and no travel powers to avoid them.
This however backfired on them a little as I was no simple newbie, nor was this my first character. So a bit of time later(I want to say, an hour?) I come sprinting back to the Atlas Park gate(totally spaced the Skyway City entrance which probably would have been closer, but I wanted them to see me run up because I knew they were still there), still alive. Didn't even use a single inspiration. Never heard a word from the douche Kinetic that instigated it. Probably a little embarrassed that his plan didn't work. His partner in crime however sent me about five team invites after I showed back up, and after declining all of them sent me a tell apologizing for their friend, he had been doing that all day and I was the first one that they felt bad for. Later became friends with them, and even roleplayed with them a few times.
Unbeknownst to me, someone else had watched my journey through The Hollows after being left to die, and was so impressed with my ability to navigate extremely hostile to my level areas that shortly after I became a Taxibot. And even after the powers revamp where you could get a viable travel power at level 4, I would still show up and offer to help wayward travelers who just didn't want to run/fly/teleport all the way back, or those who were so committed to their character design refused to take a travel power because it wasn't part of their roleplay. Besides, who doesn't like a little time saver of having someone waiting at your mission entrance for you while you hit Atlas Park(I think this was still before they had a level trainer in The Hollows) to level up for the next leg of the mission.
I think the biggest 'douche' move that I did to another was:
I honestly wasn't a mean player. I made great use of the ignore function in the game if someone was to actually annoy me. I even left one super group because one of the higher up people was the one being annoying, only to be invited in again after the person was dealt with(removed). Literally the biggest douche move I ever did was intentionally letting a team mate die in the middle of a mission. Wasn't even an important mission, just one of those side fillers. Just kinda pissed me off so, no heals for you. After about three or so deaths, I continued to not heal them but they stopped dying. So, learning experience? They stopped trying to face tank as a blaster, so win/win I suppose.