(Before reading this, i'd like to point out this is my first time posting, so prepare for a mess) So, i've been having a few errors on my server. My first is that I can't drop items from my inventory by dragging them out of the GUI. They just disappear for a split second than reappear. I haven't been able to find the plugin that causes this. (Even when I have the * permission assigned to myself, and OPed. As a side note, I can hit Q to drop normally, same with hitting Q over an item in the inventory.) Second error. Players get a message every time they want to PvP. "You can't fight someone when you are protected from PvP". I've narrowed it down to GriefPrevention. (I'm using the newest version as of this post). Me and my other staff can PvP just fine, but players can't, even in the PvP zones.
Delete "Essentials Anti-Build", it happened to me before. Just disable it with plugman if you wish. For Grief Prevention, go to their BukkitDev or just click here and read up on the configuration.
Thanks Atmazphere, the PvP problem is fixed, but the item glitch still occurs, even after reloading/restarting the server twice.