When it succeeds on the second try.
Anyone who’s done any research at all on database corruption and IO problems has probably seen an example of the 823 and 824 errors.
Error 823 is a physical IO error. It means that, for some reason, the OS was incapable of completing the requested IO. It may mean that the disk has failed, the disk is missing, a filter driver is misbehaving, or a number of other things. On SQL 2005 it looks something like this:
Error: 823, Severity: 24, State: 2.
The operating system returned error 21(The device is not ready.) to SQL Server during a read at offset 0x0000d2364e2000 in file ‘D:\Data\SomeDB.mdf’. Additional messages in the SQL Server error log and system event log may provide more detail. This is a severe system-level error condition that threatens database integrity and must be corrected immediately. Complete a full database consistency check (DBCC CHECKDB). This error can be caused by many factors; for more information, see SQL Server Books Online.
Error 823 is classified as a severity 24, the second highest severity error that SQL has. As the error message says, it’s a very serious condition.
Error 824 is a logical IO error. It means that the page requested was read from disk successfully, but there was something wrong with the page. The page header may have invalid values in it, the checksum may be incorrect, the torn page bits may be incorrect, etc. On SQL 2005, it looks something like this: