While in the inventory or the quick select boxes at the bottom of the screen, the item Stone Slab shows up as either entirely red or with the Stone Slab with a red background instead of the normal black. It is labeled as a Stone Slab while the graphic when placed is of a Stone Bricks Slab. Making a new Stone Slab or Stone Bricks Slab makes a normal items.
The only thing I can think is the item was made in an earlier version and (I'm not 100% sure) but I think what's now called Stone Bricks Slab was called Stone Slab (or the other way around) in a previous version.