We are .... Except for When we are Not - Tautology

When I was in High School, Culver Academies, I was taking AP English - Creative Writing.  My first story I got an F on, because I wrote this line:

"When I entered the dinning hall, no one was there."

Seems the professor did not know that I did not know what tautology was.

Seems I was in the dinning hall, once I walked in, therefore, saying no one was there negates my own existence, or I was wrong.  He went with the I was wrong possibility, and since that date, to this, I am careful about that particular form of logic error.  Yet, the rest of the world is not.

We're Open all the time, except when we are not open,
or ... Sunday is not a day.