During the mw:Talk:Sandbox spam attack, multiple people were deleting topics.
When I attempted to delete something already deleted (workflow rtqjfw4o2m1tfpxe), after explaining why and clicking [Delete] in the dialog I got the error box

"An error occurred.
The error message received was: Insufficient permissions to execute this action."

The API request returns

{"flow":{"moderate-topic":{
  "status":"error",
  "result":{"topic":{"permissions":{"message":"Insufficient permissions to execute this action.","extra":null}}}

}}}

It would be better to detect "Already moderated" or "Moderation state changed, reload!" or at least report the current moderation status in the error state

In case there's something additional in a server log, this happened around 20140428T13:13

(Copy of merged task T95091)

As reported User:Ace111 (before undo button has been added to wiki)
https://ru.wikipedia.org/w/index.php?diff=prev&oldid=69723086
He added to another's post his complement text. Then he tried to hide it. But it turned out that he hid the whole post. There was an opportunity to unhide, but the system would not let him do it - said that "not enough rights".

https://ru.wikipedia.org/w/index.php?title=%D0%A2%D0%B5%D0%BC%D0%B0:Senw6ztm65p3mn7j&action=history

01:19, 2 April 2015 . . Abiyoyo restored a comment
01:17, 2 April 2015 . . Ace111 hid a comment
01:15, 2 April 2015 . . Ace111 edited a comment
01:11, 2 April 2015 . . Ace111 edited a comment
01:08, 2 April 2015 . . Ace111 edited a comment
00:26, 2 April 2015 . . Abiyoyo commented

See also:
T94954 - It is not clear what Flow's hide link is for
T94317 - some Database query error

explanation:
1 user creates a post in the topic
2 user changes this post
2 user makes an action "hid" the post, and opens the history page
1 user opens the page history
1 user makes an action "unhide"
2 user makes an action "unhide" and receives an error "##Insufficient permissions to execute this action."

expected:
may be a message "This action has been done before" or similar