Basically, With changing capitalization being possible now for mojang accounts, you could change your username to the exact same name and capitalization that it was before, enabling the 30 day cooldown. I tried this on my account "ggggggggggg" and it's only visible in the API but it shows as me changing my username from "ggggggggggg" to "ggggggggggg" as you can see here https://api.mojang.com/user/profile/8a13bb92c5ac4bcbba99c9b782631eca/names. Its really easy for someone to accidentally change their username to the exact same thing as their last username and it makes you unable to rechange your username.