I can't remember how this happens, and I'm sure I used to know.

In a form, a textbox fires an "invalid date" message ONLY when run in development mode.
When built into an executable, it just hangs infinitely when the 31st of June is entered.

Yes, I know that 31/6/2004 is invalid, and so should the user (30 days have September, etc).
But something is not triggering the message in runtime, different from development.
SET DATE, SET SYSFORMATS, what is it?