Hi,
I'll make this rather short.
I booted up Minecraft and saw my skin turned into this* (*see attachment named "SkinBug" Normal skin is "Skin") The only thing I could think of that could've been the reason behind this glitch is 3D mode. I had 3D turned on when I booted MC.
I can't remember what I did to fix it. Sorry. If I remember I'll comment.
Tell me if there's more information that I need to give 🙂
Known issue with anaglyph 3D; see MC-43991.
Anaglyph recolors most textures so that you can actually see them (mostly by greatly reducing the amount of blue). Unfortunately, for some textures (mainly skins) it only does this once, and doesn't change them again when anaglyph is toggled. So if you start the game with anaglyph enabled and then turn it off, things will look weird; you need to restart the game again to fix it.
Anaglyph was removed in 1.13, so this issue no longer applies there.