What circumstances could a SQL DateTime column hold an invalid date?

I have been asked to write a series of tests within SQL. Many of them are to test if a certain DateTime columns held in the system is a valid date. Given that the all the fields in question are Datetime column and any changes made to them by end-users are through a GUI front end that already has date validation tests, not to mention that the SQL connector would through up an error if an invalid was passed through anyway. My question is that under what circumstances could a DateTime column end up holding an invalid date, i.e 20/40/2018. We are running SQL Server 2008 Kind regards Matt