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Not quite true Paul.
I have actually done it and it works reasonably well. The same is true in reverse a 1x1 flat used on 2x2 images. In CCDstack the reverse works automatically where you can use 1x1 flats on 2x2 images and it works fine.
Vignetting is a gradual thing not sharp so there are no defined hard edges which is probably why it works.
Its not an exact match so you may be left with more pronounced gradients though.
I just did a test and it worked and it even took out a dust donut cleanly but there seemed to be a more pronounced gradient (the correctly flatted image also had a gradient it just seemed a bit worse with the wrong flat).
It could get you out of trouble if you missed taking flats but its not best practice.
If you don't feel like taking flats at night you can take them during the day.
I took flats the next day with a desert storm telescope cover over the scope and a white t-shirt in front of the lens.
It worked fine. I was surprised.
I usually take flats during the day with my observatory roof closed. The interior of the room is painted matt black and I install the
soft white cover for my CDK my wife made. I make sure the scope is not directly pointing at a bright area where the roof meets the walls
and flats work fabulously. Its very convenient.
Greg.
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