This happened a couple days ago. Leveling SCH, and I've already been a bit down in the dumps due to life stresses, so I was running a few dungeons to try and get to level 66 before the night was over. Queue into Bardam's Mettle, get DRK tank, and DPS are BLM/NIN. First boss, NIN attempts to stack with me for Rush. Had no idea what he was doing and kept running away from him, but it soon became obvious....... when his BLM friend got marked, the two of them stacked with a tank and allowed the AOE to kill all three of them, leaving just me.
Me: "That isn't a shared AOE (thinking perhaps they weren't familiar with the mechanic--can't remember if there was a bonus message or not); you want to move as far away from the boss as possible, and I will shield you to mitigate some damage."
NIN: "Oh, he knows how to do the mechanic lol"
BLM: "Best party wipe I've ever done. It'll be hard to top that."
Second boss: both purposely got themselves locked out and were emote-spamming/spamming "Whoopies~" "You two got this!" while me and the DRK were doing the mechanics boss. I was already annoyed at the unnecessary party wipe on the first boss, and this just annoyed me further. Final boss, I'm honestly surprised that neither of them did something stupid to wipe the group again (though the BLM did die from standing in AOEs but meh), but the dungeon itself lasted far longer than we needed it to because of how little both of them were actually doing (caught the BLM just standing idle so much that I ended up dealing more damage FOR THE ENTIRE INSTANCE compared to them ON FREAKING SCH).
If you want to troll and goof off and cause unnecessary party wipes, that's fine, but do it in a full premade. Wish I could have commended that poor DRK for having to deal with two deadbeat DPS. I'm sure I got his comm in the end, but he left before I could select "OK" on the Commendation menu. :c To that DRK: You were a good tank, and I'm sorry we had to deal with those two clowns. Normally I just shrug off this kind of dumb behavior and ignore it, but I was so close to just leaving the instance after the first and second boss fights. If I hadn't been just a sliver away from 66, I would have. Could have probably tried initiating a kick on one of them, but I just wanted to get out of there. Wasn't in the mood for those kind of shenanigans that night.