The fact that it's a single bit confuses me. If it was a full null byte (0x00) it would be easier to explain. C strings are null terminated, so you could assume the editor stopped reading when it hit "the end of a string" but BASIC still executed it?

Or if it was some unreadable ASCII character, maybe it worked like an EOF in the editor? But the fact that he switched it from an F to an E gives me no clues. This stuff is unfortunately before my time, I'm sad I never got to play around with stuff like this in my youth.