Let's say a Wizard casts Wall of Fire in the ring shape, choosing the inside of the wall as the side that does damage.
An enemy is in one of the squares on the edge of the ring (ie. one of the 8 squares on the outside of the template for a 20' diameter circle, and not the 4 inner squares). Therefore, the enemy must make a Dex save and might take 5d8 damage.
If it makes the save, is it inside the ring?
If it fails the save, is it inside the ring?
Here is a good reference for the way the damage works: Ringed Wall of Fire: How does the damage sequence go?