While technically not a bug, there is an oversight regarding many of the textures for the GUI menus. (The latest release version still has these errors)

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 found that has this certain problem (some remnants are more minor compared to others)

textures\gui\...
bars.png
book.png
checkbox.png
checkmark.png
icons.png
server_selections.png
social_interactions.png
stream_indicator.png
world_selection.png
container\beacon.png
container\brewing_stand.png
container\cartography_table.png
container\crafting_table.png
container\enchanting_table.png
container\inventory.png
container\shulker_box.png
container\stonecutter.png
container\creativeinventory\tabs.png
advancements\tabs.png
advancements\widgets.png
textures\map\map_icons.png