After you hit the button below a post, a timer begins after which you can no longer revert that action. I don’t know how long that is, but five to ten minutes it seems.
A user created a topic and has been in the habit of “liking” () every follow-up. And then today realized that one of those posts contains content s/he finds abhorrent. When the user couldn’t unlike it, s/he flagged it for moderation, and us moderators have been unable to assist.
A little more specifics: this is a Discourse hosted install, which I will not name. I am a moderator there, not admin. The topic is for youtube videos, and post liked links to a video that doesn’t break rules, but does treat suicide in a frivolous manner. Apparently the user liked the post, then watched the video, then found the timer had run out.