See Mob Weapon Drops Before They Are Killed (Brian kopp leveling guide)

This bug is located at the Alter of Storms (and possibly other places) in the Blasted Lands. The mobs walking around there tend to have a bug with weapon drops. If the mob is going to drop a weapon or offhand, you will see them carrying it around instead of their normal default staff, sword, etc. This is very useful to scan around for epic drops, seeing as the Krol Blade and Glowing Brightwood Staff have the potential of dropping here! Thus, you can run around scanning the mobs, and if one is holding a Glowing Brightwood instead of their boring staff or sword, you are guaranteed the staff when you kill it!
This also proves that loot is pre-determined when the mob is spawned, instead of allocated once there is a corpse.