FATE: Front and Center
Location: Coerthas Central Highlands 11.3, 15.9
Frequency: Dunno, probably happens any time the FATE is left running on its own. Have seen this occur on multiple occasions. Occurring as of this post, on Mateus, 4/4, 1:40 a.m. PDT, in-game, about 7:00 a.m.
NPC: Level 34 House Durendaire Guard

Steps to reproduce: Just let the FATE run on its own. If conditions are right (may depend on exact mob spawn/position, damage, time to kill, etc), the two guard NPCs will get stuck on one another on the bridge. I believe what usually happens is one NPC is being killed and respawning, while the 2nd NPC successfully kills whatever mob it was targeting, and returns to spawn before resetting to target another mob. While NPC 2 is resetting, he collides with NPC 1 which is attempting to engage a mob. Collision and lack of AI prevents either of them from deviating, so they find themselves forever stuck together, not intended behavior.
Can be "fixed" if you draw any mob over and have them target an AoE attack near the guards. Resulting damage changes the target of NPC 2 (I think), turning him around so that both NPCs are able to resume fighting as intended.

Minor thing, but definitely not working as intended, probably just needs a small tweak to positions or AI or pathing or whatever.