SQL error severity

0 to 9: informational messages
10: informational messages that return status information
11 to 16: error that can be corrected by user
17 to 19: software errors that cannot be corrected by user
20 to 24: serious system error
25: SQL server service terminating error


http://msdn.microsoft.com/en-us/library/aa937483(SQL.80).aspx