Achievement description says "Find all the letters to Santa in the holiday missions" when it should say '10' letters. I found 7-8 or so on Santa mission one and two, then went back to mission one to complete another achievement. The letters appeared again and once I re-collected them, they counted towards the achievement and it poped. I have not beaten the final Santa mission, but I allready got this achievement.
A bug mabye?
Hope it helps.