Qala/CraftBukkit — comment on commit 5ffb00ea942666c7b54e0058f79d498488ac6067
File:  line 0
By Qala:

deathTicks is preserved across Player deaths and even saved to player.dat (as "DeathTimer").
This causes (this.deathTicks == 20) to only fire when the player dies for the first time.