This theory isn't mine, but it was too good not to share, especially since this is my favorite outfit of Usagi's.

Quote :

today’s featured outfit is usagi’s blue dress, seen throughout the series, particularly during r and s. my screencaps are from episode 66, usagi’s parental love? a curry triangle relationship.

usagi wears this outfit when she hears about the special parent and child event at chibiusa’s school, where they are supposed to make curry together. although chibiusa is intent on making it herself, usagi sees that she is having difficulty and decides to speak to mamoru about the two of them taking the place of her parents. usagi spends a lot of time with makoto and the other girls learning how to make curry so that she can do a good job for chibiusa as well as impress mamoru. as they are shopping for the ingredients, chibiusa starts to feel more and more comfortable with the pair and without thinking reaches for both of their hands, feeling the same way as she does with her parents. usagi and mamoru likewise wonder why they feel so comfortable like this.

though the outfit itself isn’t significant, the reason why i wanted to feature it is because this outfit is actually worn by minako in addition to usagi. we can see minako wearing it in episode 101, usagi in tears! glass shoes for her birthday. the fact that she is wearing usagi’s outfit at usagi’s own birthday party really suggests how close they have become. it’s never mentioned how or why minako borrowed the outfit, but usagi is seen wearing it again soon afterward so we can assume that she did return it. usagi always wears this outfit with her crescent moon necklace, but minako chooses to wear it without accessories (aside from her trademark red bow).

my theory for why minako is wearing usagi’s outfit in this particular episode has to do with what happens in the previous episode. in that episode, a boy who played volleyball with minako (who used to have a crush on her but now has a girlfriend) was targeted and his heart crystal removed. maybe after such a traumatic experience– reuniting with someone from her past, starting to have feelings for him, learning that he has a girlfriend, and accidentally revealing her identity to him– she didn’t feel comfortable returning home, or needed comfort. so she stayed with usagi, and because it was impromptu, she had no casual clothes to wear aside from her uniform, and usagi lent her the outfit. purely speculation, but i like trying to get these little things to work in canon, even if they might just be small slip ups by the animators. it’s more fun this way!

- Via shiinjite.tumblr.com

This is actually a very plausible theory. This is the only instance she wore it also, correct?

Yup Star! Though Usagi wears it several times during the series, so you could say it's a favorite of hers. It's a nice, subtle nudge to how close she and Minako are imo.

Oh I would have never noticed that! I wonder if there are any other instances of the girls wearing each others clothes? I like this theory, even if it was just an oversight by the animators. Shows don't often show small details like swapping clothes with friends and stuff (due to the whole easier to animated if you have a set outfit sort of thing) and itd be nice if this would happen more often. It's small but shows evidence of friendship development between characters.

Hm, I couldn't really find any but I would love to see it if other people have noticed some!

This is very interesting, i wonder if there's more too O: but for that we're gonna have to watch all episodes again =p