I clicked the "Thanks" button on this post and got this message in response:
A few observations:

• There are two previous "thanks" shown for this post.
• The thread is now closed.
• The original poster is no longer a member (was this what triggered the message?)
• My thanks doesn't appear on later viewings, so apparently didn't get recorded.

I can see how, if the member who posted the message has left, the system might not be able to update their "thanked posts" statistics. Couldn't that part simply be skipped, though, and the "thanks" recorded only for that post, separately from the poster's stats?