Let’s look at a couple of examples. Extract and restore the DemoFatalCorruption1 and DemoFatalCorruption2 databases, and the FatalErrors.sql script. What do we get from running DBCC CHECKDB on DemoFatalCorruption1 (lines 47-49 in the script)?
DBCC CHECKDB (DemoFatalCorruption1)
WITH NO_INFOMSGS, ALL_ERRORMSGS;
GO

Msg 8928, Level 16, State 6, Line 1
Object ID 0, index ID -1, partition ID 0, alloc unit ID 0 (type Unknown): Page (1:71) could not be processed. See other errors for details.CHECKDB found 1 allocation errors and 0 consistency errors not associated with any single object.
Msg 8906, Level 16, State 1, Line 1
Page (1:19) in database ID 8 is allocated in the SGAM (1:3) and PFS (1:1), but was not allocated in any IAM. PFS flags ‘MIXED_EXT ALLOCATED 0_PCT_FULL’.
Msg 2575, Level 16, State 1, Line 1
The Index Allocation Map (IAM) page (1:71) is pointed to by the next pointer of IAM page (0:0) in object ID 15, index ID 1, partition ID 983040, alloc unit ID 983040 (type In-row data), but it was not detected in the scan.
Msg 7965, Level 16, State 2, Line 1
Table error: Could not check object ID 15, index ID 1, partition ID 983040, alloc unit ID 983040 (type In-row data) due to invalid allocation (IAM) page(s).
Msg 8906, Level 16, State 1, Line 1
Page (1:71) in database ID 8 is allocated in the SGAM (1:3) and PFS (1:1), but was not allocated in any IAM. PFS flags ‘IAM_PG MIXED_EXT ALLOCATED 0_PCT_FULL’.
Msg 8939, Level 16, State 5, Line 1
Table error: Object ID 15, index ID 1, partition ID 983040, alloc unit ID 983040 (type In-row data), page (1:71). Test (m_headerVersion == HEADER_7_0) failed. Values are 0 and 1.
Msg 8939, Level 16, State 6, Line 1
Table error: Object ID 15, index ID 1, partition ID 983040, alloc unit ID 983040 (type In-row data), page (1:71). Test ((m_type >= DATA_PAGE && m_type <= UNDOFILE_HEADER_PAGE) || (m_type == UNKNOWN_PAGE && level == BASIC_HEADER)) failed. Values are 0 and 0.
Msg 8939, Level 16, State 5, Line 1
Table error: Object ID 15, index ID 1, partition ID 983040, alloc unit ID 983040 (type In-row data), page (1:71). Test (m_headerVersion == HEADER_7_0) failed. Values are 0 and 1.
Msg 8939, Level 16, State 6, Line 1
Table error: Object ID 15, index ID 1, partition ID 983040, alloc unit ID 983040 (type In-row data), page (1:71). Test ((m_type >= DATA_PAGE && m_type <= UNDOFILE_HEADER_PAGE) || (m_type == UNKNOWN_PAGE && level == BASIC_HEADER)) failed. Values are 0 and 0.CHECKDB found 5 allocation errors and 3 consistency errors in table ‘sys.syshobts’ (object ID 15).
Msg 7995, Level 16, State 1, Line 1
Database ‘DemoFatalCorruption1′: consistency errors in system catalogs prevent further DBCC checkdb processing.CHECKDB found 0 allocation errors and 1 consistency errors in table ‘ALLOCATION’ (object ID 99).
CHECKDB found 6 allocation errors and 4 consistency errors in database ‘DemoFatalCorruption1′.

A bunch of errors that look like regular DBCC CHECKDB output – but if you look carefully near the end of the output you’ll see error 7995 stating that the system catalogs are so corrupt that DBCC CHECKDB can’t continue. Notice also that there’s nothing at the end of the output stating what the minimum repair level is to fix the errors – because repair cannot be run on this database.
The second example is even worse (running lines 53-55 in the script):
DBCC CHECKDB (DemoFatalCorruption2)
WITH NO_INFOMSGS, ALL_ERRORMSGS;
GO
Msg 211, Level 23, State 51, Line 1
Possible schema corruption. Run DBCC CHECKCATALOG.

In this case, the corruption is so bad that DBCC CHECKDB didn’t even get a chance to terminate gracefully – the metadata subsystem in the Query Processor just blew away the whole command. Running DBCC CHECKCATALOG as the error message states doesn’t do any better – it just prints the same error! (I didn’t write that error message :-)
So – just because DBCC CHECKDB completes, doesn’t always mean it completes successfully. Make sure you always check the output.