I know that new content releases always come with bugs, but I was still surprised to run into what I would consider a pretty game-breaking bug today. My pet tank and I went back to do the level 70 solo instance in Neverdeath Graveyard that my great weapon fighter struggled with (the quest is called "Ghost Stories"), and as a duo we did alright, even if the scaling didn't work too great. However, just as we entered the last room of the instance, the level scaling simply fell off - leaving our level 60s to face off against level 73 mobs with no bolstering whatsoever. Unsurprisingly, we couldn't even scratch them. This made it impossible to finish the associated meta quest - not until we're level 70 anyway. I found a forum thread about the problem, but there is no known workaround.
In less game-breaking but still annoying news, one of the quests at the Drowned Shore refused to update properly for us after we had already fulfilled its requirements. Fortunately the forums came to the rescue again, this time with the advice that the quest only bugs out in a group and can be fixed by leaving the party. Still annoying though.