The story begins in Gnomeregan, I was on my rogue alt and I thought my group took pretty weird and long ways so... I thought wtf - let's solo a boss, when they cleared all mobs on the upper deck of the chamber before the path to the final chamber, they went on some weird sideway and I sneaked past everything to the boss that drops Workshop key - solo'd him easy with Thisteltea & a pot. Easy. I was getting hungry IRL so I thought - let's just get this done, I did manage to sneak to the final chamber alone and not draw any aggro - open the door and vanish.
I did pull the boss, I was 95 % sure I would die. But I thought - lol let's give it a try -. Of course I wiped. Got him to like 40-50 % hp, but I got the message *Gnomeragan complete* You receive this amount of xp to complete the instance and this Satchel of Helpfull goods.
I thought this can't work as intended, tried to find any proper bug report forum on the EU side but can only find one on the US one and I can't post there!
I've had something different, but definitely buggy as well.
I was in Nexus (regular) on my mage in a group when we got all the way to the final boss. Before the pull, and even before that final mob you have to kill, a friend messaged me wondering if I wanted to get in on an ICC run, but I needed to switch right then and there. So I alt-F4ed out, came back in (this process is quicker than waiting the 20 sec to log) on my 80, and did the ICC run. After ICC, I went back onto my mage to port him to an inn for the rested bonus. As soon as I logged on, I got credit for the dungeon/experience/gold etc.
I've had something similar to this before when i queue'ed up right when i accepted it flashed instance complete across my screen and i watched everybody say 'thx for group' then leave.