Pull a series of CraftBukkit patches - see below for summary and credits:

Bukkit/CraftBukkit#1145 : @Zarius : Don't fire piston extend event twice
Bukkit/CraftBukkit#1164 : @millerkil : Prevent plugins causing ghost players
Bukkit/CraftBukkit#1177 : @jb-aero : Properly set ambient flag for potion effects