Java Edition:Locked Chest
Locked chests were an April Fools' joke for 2011. Unlike other April Fools' updates, this feature was directly put into a normal Beta release rather than making a different April fools' version. Upgrading locked chests to 13w41a (1.7 snapshot) and later converts it into white stained glass.
Although the item does not exist beyond 13w41a, it can still be proven that a world had the item because discontinued NBT that was only obtainable before stained glass exist will convert onto the glass. For example, a nameless or enchanted locked chest will convert those onto the white stained glass.
Obtaining
Beta 1.4
In Beta 1.4 (including original and reupload), locked chests sometimes generate in new chunks in a random location with a solid block below and an air block above. While one most easily finds them on the surface, they can also spawn below ground in cave systems. However, this will only happen if the computer clock is set to April 1st of any year. Attempting to mine the locked chest would not work, because it was set with a hardness of -1. However, explosions from TNT or creepers will destroy it, as it was not set with a high blast resistance. It will have a 1 in 4 chance to drop it as an item from TNT, or a 1 in 3 chance from creepers.
One may also update a world with locked chests placed into versions past Beta 1.4, and gain the ability to instantly mine the chests, but locked chests will also disappear whenever they are randomly ticked, similar to leaves.
Beta 1.6 Test Build 3 Debug Chests
In Beta 1.6 Test Build 3, any newly generated world would contain up to 11 debug chests with every item in the game. The locked chest item was obtainable in the chests 4th and 7th from the left in the 8th and 21st slots respectively, in stacks of 64.
Texture Changes
Beta 1.4 - 12w19a (1.3 snapshot)
12w21a - 12w21b (1.3 snapshots)
12w22a (1.3 snapshot) - 1.4.7
13w01a - 13w02b (1.5 snapshots)
13w03a (1.5 snapshot) - 13w17a (1.6 snapshot)
13w18a (1.6 snapshot) - 13w36b (last version)
Properties
When placed in Beta 1.4_01 or later locked chests would "decay" in a similar fashion to leaves, deleting it permanently from the world. In Beta 1.8.1 (likely other version as well) each locked chest has a ca. 0.224% chance to be deleted on each tick, giving them a half-life of 283.9 game ticks (14.1 seconds). If a player breaks a locked chest with their fist before it decays, the block will drop as an item.