Does a player win Catan if they forget to count their points?
The True Winner is player A.  The first player to obtain 10 points on his or her turn is the winner.

From the online FAQ about this situation at catan.com


  The game ends as soon as a player has reached the required number of victory points on his turn. If he’s unaware of it, you should tell him, because you can’t take an already gained victory away from him.


This means my earlier comments to the contrary are WRONG, please ignore them as the rantings of a madman. :)