The doors dent extremely easily in my experience! There's a lot of exposed slab sided metal on those doors and I don't think it's particularly resilient to knocks. Unfortunately I've had to employ the services of a PDR guy a couple of times. Once was my own fault where a bit like the OP, I let the door open against a window sill and managed to bend the edge slightly.
Luckily my PDR guy is very good and neither ding is visible now.