What circumstances could a SQL DateTime column hold an invalid date?
SQL Server will not allow an invalid date. Internally, the date part of a DateTime column is just a number/four bytes that represents dates between January 1, 1753, through December 31, 9999.  SQL Server does not know anything about and will not use invalid dates. An attempt to save an invalid date will result in an error.  

If you want to confirm this, write some code that attempts to store all possible number combinations for the digit groups in a column.

It is possible, however, to inadvertently store the wrong date, due to SQL Server DateTime supports multiple formats, however that should be handled by your business logic layer. For example, someone may enter 07/06/2018, intending the date to be June 7. In most cases this can be avoided by using ISO 8601 DateTime format when a DateTime needs to be represented as a string.

https://en.wikipedia.org/wiki/ISO_8601