I have recently switched to using my left hand as the main hand in Minecraft. This is all handy-dandy until I realised that disabling the first-person skin model reveals an offset outer layer, as shown here:

It is not a skin issue (for reference, pictures of my skin are posted at the bottom)
Sure, it's only annoying to left-preferring players and wouldn't ever be that high-priority but it's something that's very easily overlooked.
Also, while writing this, I discovered that the first-person hands also don't reflect the skin customisation options; I had all my outer layers except for hat turned off. Still not any major bug that needs to be fixed or anything.
Left hand (with markings):

Even Right hand (with markings):

I am using other mods but they wouldn't affect the player model in any way.
Fix in your own time, no rush.

My skin (visualised on minecraftskinstealer.com):



In MC using this mod:

Fixed the left hand in the next update.