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:
- Backup all the user and system databases (with checking integrity of the backup)
- Check the health status of the user databases (DBCC CHECKDB)
- 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!