Last night when I beat the game and need one last achievement to total the game 1000 points but it read that I need 2 achievements!
It is a bug in the code and only allows you to get 990 points.
To fix this (until Bungie put out a fix) you have to recover your gamertag.
You may find this happens to you when you unlock 3 achievements all at once.
To recover do the following...
1. Sign Out
2. Select Recover gamertag from menu
3. Enter details of your account
4. Your gamertag will be recovered with the bug now fixed.
Note: If you earn offline achievements these will not then be there between the last online session and your recovery.
