Why do I get an error "Msg 602, Level 21, State 3. Could not find row in Sysindexes for dbid 'x', object 'yyyyyyyy',index 'z'." when running a SQL Server query?

A. First of all it could be a corruption exactly like the message says. Run a dbcc checktable against sysindexes on the database concerned. Also a dbcc checkcatalog.

If these return clean then it is a SQL bug. There are lots of bugs and fixes for 602 errors. Look for reasons for this bug at http://support.microsoft.com - just search on 602 and SQL and kbbug. If your problem does not match any of these and you have already applied the latest service pack then either post a reproduction script for an MVP to report for you or call the problem in to Microsoft PSS yourself.