Arena Glitch Chest.

[Даркфайт]

There's a bug/glitch with Darkmoon Arena chest, also Gurubashi and open-world chests from vanilla to cata.
Normally it can be used by 1 person, but if that person keep some items inside other people can loot full chest.

 

[Calligraphy]

Not quite. Most of the items can only be taken by one person with the chest despawning only when it's empty, but everybody can loot a copy of the trinket as long as the chest exists. At least for the arenas. I also don't know how unintentional it is, since if you can get people cooperating so everyone gets a tag that looks like a positive for cooperation in a MMORPG.

 

[Editer]

I guess it should work with chests in dungeons too, like https://www.wowhead.com/object=74448 or https://www.wowhead.com/object=74447 etc.