This bug revolves around the following command, and incarnations of it:

/testfor @a {Inventory:[

{Slot:103b,id:"minecraft:diamond_helmet"}

]}

Using this command sometimes runs succesfully, but most of the time fails to succesfully find an entity that matches it.

I've had two different cases:

1) A player with a diamond helmet equipped doesn't give a true output. (Strangely, this didn't work when the player shift-and-left-clicked the helmet from their inventory, but did when it was dragged in manually. This might be a different bug entirely, though.)

2) Changing the scoreboard of an entity with a similar data tag also gives a false negative. An armor stand with wool on it's head doesn't come back positive when the command specifies wool rather than the diamond helmet.

This command is run on a repeating command block and doesn't work in 1.9.