I too, had my first serious ankle sprain riding flats 2 years ago (my first year riding flats)I was on flats. Sometimes sh*t happens.
I put my foot down. Only the spot where my foot landed had some fluffy leaves that makes it LOOK quite a bit higher than reality, plus some rocks under the leaves that started to move as soon as my foot landed on them...
The benefit of flats is you can put your foot down when in doubt. But the danger of putting a foot down is added risk of sprain ankle.