any old school fan remembers in daggerfall when you play a female character, in the character menu you can disrobe her completely. it was so cool seeing nvde orcs, elves in addition to humans.
but in skyrim you can NOT go fully nvde. suprising since you can bash the heads of almost every living thing in skyrim but you can only see characters down to their brown furry bikini covers.
no explanation will justify this misstep.
imagine the total coolness factor spike up if there was a quest where you disrobe a character and they have to run through town nvde, or a quest with a Crying game situation. we americans need to get more with the europeans who walk around in nvde beaches and are not so uptight about being nvde.

Its been that way since Morrowind
If you are on PC there are mods that remove or change the underwear

I wouldn't call it a mishap.

I did notice that NPCs sometimes comment on what you're wearing
1 time I grabbed all my gear from a chest but forgot to put any armour or clothes on and a NPC said something about not thinking much of my outfit if thats what I called it

No amount of gore can possibly give the game an advlt Only rating. A single exposed female nipble on the other hand...

The only naughty bits you are allowed to see in this game is the stump of someone's neck after you decapitate them.