Java Edition:Open Barrel
When updating out of the April Fools version 3D Shareware v1.34, previously opened barrels display the texture of an open barrel
Obtaining
Any barrels that are opened during this version display the texture of an open barrel after updating. There has to be an item inside a barrel for this to work, otherwise the barrel will explode. In order to get an empty barrel the item placed inside could be exclusive to version 3D shareware, such as the red, yellow, and blue keys, or a hopper could draw items out of the barrel after updating. Using a hopper in 3D shareware still causes the barrels to explode. This doesn't work if updated to snapshot 19w14a or 19w14b, thus any discontinued features during those two snapshots are mutually exclusive to open barrels. Placing and breaking blocks around the barrel doesn't update it, however opening the barrel in a later version will cause it to reset it's texture.
Variations
Any combinations of items could be contained in them, including discontinued ones. This allows for millions of variations. However, all of these variations are considered unobservable discontinued features since you can't tell which items are in these barrels without the use of commands. Barrels can also be placed facing any of the cardinal directions, as well as up and down. The barrels can also be combined with Comparator With Signal Strength Greater Than 15 and Comparator Output From Empty Container.