Canonically? Where?
The only thing resembling actual "unobliviating" that I can think of is Lockheart apparently slowly slightly recovering from his memory charm blowing up in his face via the broken wand. And that was after several years of ongoing treatment at St. Mungo's.
Bertha Jorkins. After she found out that the Crouch family was keeping Barty Crouch Jr. imprisoned in their house, Crouch Sr. put a Memory Charm on her strong enough to cause her permanent brain damage and forgetfulness. But Voldemort was able to break through it with torture.
Update: This post has also been superseded - new comments belong in the latest thread.
The second thread has now also exceeded 500 comments, so after 42 chapters of MoR it's time for a new thread.
From the first thread: