I've been streaming some GW2 story recently, so I caught film of this, which should help a lot in finding it. It's pretty funny.
SPOILERS: This bug was found in a story instance for Asura - specifically the level 20 plot if you choose Val-A golem at chargen and then choose to investigate Keppa later.
Here's a link to the video:
For Details of bug for investigators, I caused this on September 12th, Server time can be seen in the video on the minimap.
Character name Leggia: Asura, She is level 80 warrior/berserker, Story instance (Val-A story, catching up with Keppa at Skrittsburgh) No group, player server is Ehmry Bay.