When a character is not wearing (a) Hair Ornament(s) they usually wear, but said hair ornament(s) is/are still present in the picture. If a character has not removed all their hair ornaments, it still counts, as long as at least one of the removed hair ornaments is still present.
Something like post #6576289 does not count, because in that post, Chiaki is still wearing her spaceship hair ornament. The artist just made the stylistic choice to include multiple representations of it in the picture.