You should be carefull when apllying changes to the producion system. This is especially true for applying Service Packs. The main steps before patching SQL Server are:

  1. Backup all the user and system databases (with checking integrity of the backup)
  2. Check the health status of the user databases (DBCC CHECKDB)
  3. Check if this specific SP has some issues, bugs, Connect items

If you dont followi the steps it is not necesary to have a problem. But statistically it happens at some time.

Like in this case, when I am shaing my experience in applying SP2 of SQL Server 2008 R2: http://maginaumova.com/master-database-error-after-applying-sql-server-2008-r2-sp2/

Good luck and thanks for reading! :-)