In Wikipedia it is possible to send other users Thanks notifications (see https://en.wikipedia.org/wiki/Wikipedia:Notifications/Thanks ). If you do this the button will change from "Thank" to "Thanked". However this is not always the case. I have come across multiple instances of Revision history pages or diff-pages ("Difference between revisions") where I know I have already thanked a user for a specific edit but the button is still shown as "Thank" while it should be shown as "Thanked".
Unfortunately I wasn't able to reproduce this error yet nor have I any idea what might be the cause of this.
I have not been able to reproduce this error yet.