So, more time now.
NBTExplorer, open the level.dat
Go to Data->Player->Inventory
All the entries there describe, what you have in your inv.
If you know which item makes the problem (and where in your inventory it is):
Slot 0-8 is the hotbar, 9-35 is the inventory, 100-103 is the armor.
If you only know the item ID: look at the id-entry (and damage-entry for metadata).