When you get all achievements for a given game and the developer then adds new achievements to the same game, the cover card for this game in the library will still say "100% complete" even though that is not the case anymore.

This will fix itself by launching the game, but there is no way to see the issue without manually clicking a game in the library and noticing that the count is actually not 100% any longer.

Steps to replicate :

1. Get 100% achievements in a game
   
2. Developer adds new achievements to the game
   
3. Result : game cover card in library still says "100% complete"