"Is it not a good idea to do this by default, so the error message does not perpetuate ?"
No necessarily, that's a bit like saying "shouldn't all cars have automatic transmissions".
I personally prefer manual, but i know many people that don't.
The errors will be cleared by 2 events:
- Submit the form successfully
- Close the browser to end the session.
if you chose, you can unset the session manually:
<?php if(isset($_SESSION['validationSession'])) unset($_SESSION['validationSession']); ?>