Using 4.3.4
We have a strange problem. If someone adds an item to their cart, then goes to My Accounts and tries to login, but they provide a wrong login (i.e. a typo), they are getting the store closed message and graphic.
What would cause this strange behavior and how to fix?
Hm.. Maybe clear the cache?
The fix is not that easy. I’m not a newbie 
Of course I cleared the cache.
The fix is not that easy. I'm not a newbie :)
Of course I cleared the cache.
Is the Service unavailable banner displayed? If yes, please press CTRL+U on your keyboard, scroll down to the end of the page with source code and provide us with the error which is displayed there.
Unable to provide any data, as soon as a do a view source or reload the page it correctly goes to the login page and says wrong username or password page.
Note, the url never changes. It still shows https://www.homeschooldiploma.com/login/?return_url=index.php, but the content in store_closed.html is showing until one reloads the page. Oddly this only happens if one has an item in the cart.
Failed to reproduce the issue according to your instructions (cookies are cleared)
If you are familiar with the code inspector of your browser you can also check the error message on the bottom of the page without page reloading
I was able to trace it down a little. It appears the showStub routine in Development.php is getting triggered with a invalid login?
Unable to provide any data, as soon as a do a view source or reload the page it correctly goes to the login page and says wrong username or password page.
Note, the url never changes. It still shows https://www.homeschooldiploma.com/login/?return_url=index.php, but the content in store_closed.html is showing until one reloads the page. Oddly this only happens if one has an item in the cart.
We also failed to reproduce the issue in your store. Maybe you missed something else in your instructions?