From Wikipedia:
"Diacetyl (IUPAC systematic name: butanedione or 2,3-butanedione) is a natural byproduct of fermentation. It is a vicinal diketone (two C=O groups, side-by-side) with the molecular formula C4H6O2. Diacetyl occurs naturally in alcoholic beverages and is added to some foods to impart a buttery flavor."
In "edible" form, it is fine... but in vapour/gaseous form with long term (no one really knows how long) it is toxic and may cause Bronchiolitis obliterans, which is an irrervisible and potentially fatal lung disease.
So.. of course we are heating and vapouring juices.. so juices that contain this ingredient are avoided like the plague. There may only be very minute or trace levels in the juices that contain Diacetyle, however as with anything you are inhaling, it is best to be safe than sorry.
A common product to find this substance in is Butter-flavoured popcorn. Ingestion is not a problem but microwaving it could be.
