Arithmetic overflow on SELECT query
This can happen in several ways, for example as described in Troubleshooting DBCC error 2570 in SQL Server 2005 and later versions:

Invalid or out-of-range data may have been stored in the SQL Server database in earlier versions for the following reasons:

Invalid data was present in the source while using bulk insert methods, such as the bcp utility.
Invalid data was passed through RPC event calls made to SQL Server.
Other potential causes of physical data corruption left the column value in an invalid state.


That article contains a lot of useful information on the topic. For the basics, see the documentation for DBCC CHECKDB and the DATA_PURITY option in particular.