Bukkit/CraftBukkit — comment on commit 696ba64d7ef779da1cbeafef747b30c29be672f6
File:  line 0
By andf54:

Projectile fires off two EntityDamagedByEntity events. First events .getEntity() gives the projectile, the second events .getEntity() gives the creature/player that shot it.
There is a problem if you want to modify all non-projectile events, because projectile events will also be affected.