I was developing against SQLite for a while and didn’t notice that I had accidentally put "NULL" (i.e. as a string) in the fixtures. SQLite didn’t care about this, but MSSQL certainly did. Because I specified constraints on the column widths, I got the error:

ActiveRecord::StatementInvalid: DBI::DatabaseError: 22001 (8152) [FreeTDS][SQL Server]String or binary data would be truncated.

At first I thought you had to use nil in fixtures, but in fact, it’s actually NULL that you want.