When you do the /clear command to remove the items from your iventory, it says "An unknow error occurred while attempting to perform this command" but it cleans the inventory too.
EDIT:
It only give this message when the player is wearing some type of armor
Duplicate of MC-44556 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.