He's probably always forgetting his underwear in previous locations of undress. Usually, it seems to be McCoy's facilities. That K/S thing is just a smokescreen...
Now, for some

's advocate stuff on a well-explored subject:
THE CAGE: All officers have one solid gold stripe and Senior ratings have one gold stripe with rectangles cut into it.
Or then there are no senior ratings in evidence, and the perforated stripe has some other meaning (say, the usual Lieutenant, Junior Grade). After all, while "CPO Garrison" is specified as Adam Roarke's character in the end credits, neither the name nor the rating are actually part of the dialogue, and end credits aren't really part of the Star Trek universe as such.
Everybody wearing the solid stripe could be a Lieutenant; we know that Number One is, at any rate. It would be a bit odd for Captain Pike to be a Lieutenant, though.
WHERE NO MAN HAS GONE BEFORE: All officers have one solid gold stripe, the captain has two solid gold stripes and Senior ratings have sod all.
Unless the new movie contradicts this (that is, shows Kirk promoted to full Captain before the events of this episode, and still somehow fits into the TOS continuity), we could argue that the Captain's rank here was Commander, therefore compatible with his braid in the TOS sense - and Lieutenant Commander Mitchell had either been very recently promoted, or was promoted posthumously, hence the lack of the "half-braid" to accompany his single stripe.
TOS (all the other episodes):All ratings and ensigns sod all, Lt (JG) one broken gold stripe, Lt one full gold stripe, Lt Commander, one broken and one full gold stripe, commander two full gold stripes, captain two full gold stripes and one broken gold stripe, fleet captain never shown, possibly three full gold stripes?, commodore one thick full gold stripe. Admirals ranks never shown.
Fleet Captain need not be a rank as such, considering the ambiguous one-and-a-half references to it. And the Admirals did have braid on their sleeves, even if it didn't show up on camera; the famous gag reel bit on Ed Reimer / "Admiral Fitzpatrick" shows his sleeves with one broad strip and two narrow ones, flanking the broad one from above and below, and perhaps indicating a rank two steps above Commodore (that is, Vice Admiral).
Timo Saloniemi