I've seen quite a lot of these used as legitimate plot devices. So although the thread takes it to the extreme, many of these can be tweaked to give your mission something different without being annoying.
Vanden has a pretty unique arc that has similarities to some of these tricks, if you want an example.
New idea!
Fill the map with standard mobs, give them powersets that won't be overly annoying. Just when the team running the arc is getting comfortable, turn those mobs up a notch! Give them extreme/extreme powers, but make sure they look exactly like the other mobs, even down to the same name.
This can be done from one mission to the next, or all in one mission if you add a boss fight or destructable object in the back area. Getting them in a group with the same name might be tricky, but remember, uppercase "i" and lowercase "L" look the same in game. 
(I know you can have two critters with the same name by changing the save path, but I don't know if you can do that with groups)
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Is "APATHY QUIT!" a meme yet?
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It is now! Thanks! 
Ran into one yesterday that I think was unintentional. There was an ally by herself in the first room, so there was no way to avoid picking her up. She was set to aggressive, had long range attacks, and apparently increased perception. Throughout the entire mission the ally would pull from a different group on each shot. Since she was a minion, she didn't contribute much DPS, but boy did she try to grief us!
Now imagine a large map with long site lines, such as a warehouse, with several allies like that. Put it at the end of a multi-mission arc so the player has some time invested in the arc...
I ran into an interesting trick today. I had to rescue hostages on a rather big map. The problem was the hostages were dressed exactly like their guards. So much for zooming over with flight and searching for the hostages (and, yes the hostages were standing up and doing nothing like the guards).
If Brevity is the Soul of Wit, Why are You Reading This?
Also, more of a bug than a trick, but escorts will not work in certain outdoor maps like the Forests. You can lead the escort in and out of the orange glowy box of freedom, but they'll never leave. It seems the way-point is 50 ft underground.
Definitely frustrating! It's a good way to completely halt any progress in your arc.
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I ran into an interesting trick today. I had to rescue hostages on a rather big map. The problem was the hostages were dressed exactly like their guards. So much for zooming over with flight and searching for the hostages (and, yes the hostages were standing up and doing nothing like the guards).
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I guess my heroes are better than most at detective work .. they can fly over and immediately identify friendly targets.
/bind [key] "target_custom_next friend" 
How about this for cruel take a large defeat all map custom make every mob with endurance drains + sappers and anything else that sucks endurance set up glowies and and in a couple elite EB or AV's with short circuit and power sink
I mean just lay on the -end on every mob
I mean people just love fighting all that -end mobs
Pinnacle
Langar Thurs-Katana/SR 50; Hejtmane-DM/DA 50
Rogue Spear-Spines/DA 50; Hypnosis-Ill/Rad 50
Sir Thomas Theroux-DM/SR 50; Melted Copper-Fire/Shield 50
Byzantine Warrior-DB/ELA 50;Blade Tempo-50 DB/EA
Make an awesome looking high volume easy enemy farm, however set the objective to "Ally - Single Target". As soon as the first person enters the mission ends, no one else can get in.
Liberty:
proud member of the LEGION, SISTERHOOD
@Doctor Photon
No need to trick the casual player when 90% of everything is farming. But as annoying others seems to be a hobby with so many have at it.