The ATM said "Please insert your check here, face up, with the top of the check to the left".
I did so. It spat it out, saying "Try again. You must insert your check face up, with the top of the check to the right."
It went back to the original screen, saying "Please insert your check here, face up, with the top of the check to the left".
Not knowing what to do, I inserted it with the top of the check to the right.
And then it worked!
I remember there's some aphorism about stopped clocks...