Chummy delivering the breech baby completely on her own and without freaking out once. It's notable that she uses a technique which was described as dangerous by Sister Evangelina, but the baby is born alive.
Trixie and Sister Evangelina on the ship.
Chummy delivering triplets with hardly any supplies, even to the point of using her nursing uniform as a blanket.
Dr Turner, Sister Bernadette, and Trixie delivering twins, one of which was lying transverse and had to be turned by Sister Bernadette, without losing mother or either baby, despite premature placental separation and uterine hemorrhage.
Dr Turner and Sister Bernadette convincing the board to send a tuberculosis screening van to Poplar, which desperately needs it.
Jane making up for a lack of judge in the baby competition by deciding at the last minute to run on stage and announce Sister Monica Joan as the replacement. Considering how much of a Shrinking Violet Jane had been not so long ago, her addressing a large crowd of people is pretty notable.
In the next episode, she is able to press Fred into facing his fear of heights and climbing a ladder to get Sister Monica Joan out of the bathroom ("Normally I'd be sympathetic, Fred, but today I'm not"). Actually, pretty much anytime Jane is assertive could count.
Dr Turner coming back from a nasty resurgence of his shell shock just in time to save the life of a pregnant mother with diphtheria via emergency tracheotomy.
After almost a full season away, Sister Evangelina's voice is heard from beyond a pile of gale-induced rubble...and she's scolding the workers for panicking about rubble instead of fixing it!