How can I corrupt a table in SQL Server?
Paul Randal has several Sample corrupt databases to play with:


DemoDataPurity -  192-MB SQL Server 2005 database with a single 2570 (data purity) error 
DemoFatalCorruption1 -  1-MB SQL Server 2005 database with a corrupt system table (that allows CHECKDB to complete) 
DemoFatalCorruption2 - 1-MB SQL Server 2005 database with a corrupt system table (that terminates CHECKDB) 
DemoNCIndex - 192-MB SQL Server 2005 database with a bunch of nonclustered index corruptions 
DemoRestoreOrRepair - 1-MB SQL Server 2005 database with a page checksum failure (in fact a zero'd out page) 
DemoCorruptMetadata - 1-MB SQL Server 2000 database with corrupt syscolumns table 


Also, How to create a corrupt database using BULK INSERT/ UPDATE and BCP - SQL Server as a HEX editor.