I respectfully request that players be disallowed from accepting a "BG Que" or "Summon" while 'In Combat'.
Players have regularly, and continuously abused this system to bug out flight master NPC's, preventing the NPC's from returning to their spawn points -- at which point, these NPC's will keep their threat and *not* path back to their spawn points. This is something people maliciously use 'Group Que' for... having a friend back in Ogrimmar, or the city of your choice-- while kiting flight master NPC's away.
These NPC's NEVER respawn, because they aren't dead.. but when they're off their spawn point? They cannot be used for flight path, even though they're out of combat.
At the very least, I recommend this as a temporary measure until the threat bug can be fixed.