If you try adding custom NBT data to an item and have unbalanced curly brackets ({), it'll say you have unbalanced brackets. If you put unbalanced brackets ([), it'll say you have unbalanced curly brackets.

My testing:
Working:

/summon Arrow ~ ~ ~ {Motion:[1.0,0.0,0.0]}

Unbalanced "Curly":

/summon Arrow ~ ~ ~ {Motion:[1.0,0.0,0.0}

Unbalanced "Normal":

/summon Arrow ~ ~ ~ {Motion:[1.0,0.0,0.0]

And

Working:

/give FlyingPies155 1 1 0 {display:{Name:Test}}

Unbalanced "Curly":

/give FlyingPies155 1 1 0 {display:[Test}

Unbalanced "Normal:

/give FlyingPies155 1 1 0 {display:{Name:Test}

I know the /give example for the Unbalanced "Curly" is not how you actually use the metadata thing, but it still works.