Now, errors like that are easily rectified. What I'd normally do is immediately say in party chat that I didn't mean to need the item, and offer to pass it to anyone who did. I will check through the loot rolls to see if anyone else needed it, and if not I'll probably try to pass it to anyone who can wear it! I always feel dreadful when this happens, so I almost prefer to give the item away to someone who can't use it to preserve my integrity as someone who isn't a ninja.
In the past, I've been an accidental ninja where I've failed to understand correctly what stats I need for a given class or spec. You know, needing on agility mail on a sub-40 paladin when there's a hunter in the group because I'd heard somewhere that agility gave me dodge.
Or I've just misread the tooltip, or read it too hastily. Hitting need on spirit cloth on my shaman, for example, just because I saw the spirit on there and I was healing.
How about you? Have you ever been an accidental ninja? Or a deliberate one?