Dualcore DBA demonstrates how you can turn an outer join into an inner join on accident:
This post is a quick public service announcement to serve as a reminder to be careful with joins in SQL, particularly in queries with a large number of INNER and LEFT JOINs.
An INNER JOIN onto a LEFT JOIN will effectively make that LEFT JOIN an INNER JOIN.
This is a bit different from the other major case: a LEFT OUTER JOIN whose column you use in the WHERE or HAVING clauses.