We guessed detail about the trouble.
1) He pinned toot<A> on profile.
2) He deleted toot<A>.
3) Toot<A> was deleted, but the number of toots that can be pinned --- the status of "pinned toots" wasn't undo.
4)He repeated 1)～3).
5)As a result, space for possible pinned toots was decrease to 1.