(Resubmitted due to mislabeling affected version as future releases)
While technically not a bug, this is more or less an oversight regarding many of the textures in game, specifically the GUI menus.

When I was making a resource pack, I discovered that a bunch of the GUI menus textures (as well as others) have transparent pixels that are very hard to catch that seem to be remnants of when the textures were originally being created.

For example, the stonecutter's container gui has remnants of the loom's container gui in it's texture, just very transparent.

If it's important, I used paint.net and a plugin for it called "Alpha_to_255" which is how I was able to view the remnant transparent pixels.

Here is a list of the textures that I have found that contain hidden remnants in their images (some remnants are more minor compared to others, and list likely does not contain all examples of this texture issue)
textures\gui\...
book.png
icons.png
server_selections.png
social_interactions.png
stream_indicator.png
checkbox.png
world_selection.png
container\crafting_table.png
container\brewing_stand.png
container\cartography_table.png
container\enchanting_table.png
container\inventory.png
container\shulker_box.png
container\stonecutter.png
container\creativeinventory\tab
advancements\tabs.png
advancements\widgets.png

textures\map\map_icons.png

The attached GIF shows examples of some of the texture remnants.