Bukkit/CraftBukkit — comment on commit 032d02803846834304edc262d41b2786b5ae995e
File:  line 0
By Moo0:

Without a stacktrace or anything here, this message just spams the log without providing much information as to what is causing it.
But, I have noticed it happens very frequently for certain players, after they have died, but not for most players, so I'm thinking it may be client-related. A cheat-type client mod sending packets of some kind abnormally?