I'm not sure about that's a bug, but i wanted to report it. While i was investigating some NBT keys at "http://minecraft.gamepedia.com/", i have found a code "/summon Item ~1 ~0 ~1

" then i wanted to try it. After typing this, i'm getting this error: "An unknown error occurred while attempting to perform this command". Only "/summon Item ~1 ~0 ~1" summons a stone (probably being it's first item) but when i bring this NBT key near on it, it gives error. Absolutely: there were no mod or plugins while i testing this command.

If that is not a bug, sorry

Source: http://minecraft.gamepedia.com/Tutorials/Command_Block