If I click the lock before a locked thread’s title1, a pinned thumbtack appears.
If I keep clicking the lock or thumbtack, the topic will toggle between pinned and unpinned (or at least it looks like it, judging by the state of the thumbtack).
There’s no such toggle for unlocked threads. I’m not sure which of these two cases is in error.
1 A thread I can’t unlock. I haven’t tried otherwise because I don’t have enough of a trust level.