/blockdata ~ ~ ~ {Items:[{Slot:0b,id:minecraft:air}]} Deletes every item. I used this command In a Furnace to delete one slot but all the other slots get cleared too.

Intended, to only change 1 item, use /replaceitem
Also please don't make the title the summary, the title of the report is a few words discribing the bug, the summary may have all the details you need to reproduce the bug