This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Mary Poppins
Michael's turn carrying the Jerkass Ball climaxes with him swiping Mary Poppins' magic compass. Instead of naming the four directions one at a time, he quickly says all of them in a row. This causes everyone that Michael and the others met during their first journey with the compass to surround him with bloodthirsty expressions.note In the 1934 version, the racial stereotypes direct weapons towards Michael. The sight scares Michael into dropping the Jerkass Ball by the time Mary Poppins swoops him back to the nursery.
Mrs. Corry breaking off her candy fingers and feeding them to the children comes off as this to readers who see it more as Body Horror than a hospitable act.
The Holy Terror herself, Miss Andrew in a nutshell.
After Jane takes out her frustrations at being the eldest Banks child by punching Michael, Mary Poppins forbids her from attending a tea party at Miss Lark's house. Home alone, Jane pays a visit to some boys who reside in the design of an antique Royal Doulton bowl. When she asks to go home to her family, the boys' great-grandfather tells her that when she entered the bowl, she also became teleported 60 years into the past. With her parents not yet born, it looks like Jane must live as a Fish out of Temporal Water, and the youngest sister of boys who are Older Than They Look...until Mary Poppins whisks her back into the real world. Upon returning to her own time, Jane wholeheartedly accepts her responsibilities as the eldest Banks child. Mary Poppins Returns includes this bowl, but in a muchLighter and Softer turn, it just leads to a world of singing cartoon animals.
Michael has a similar experience in one of the other books, after journeying to a star inhabited by talking cats. He becomes seemingly unable to return home because time passes more slowly on the star than on Earth, and during the time he spent with the cats, everyone in Michael's family died. Their house also became abandoned. Fortunately, Mary Poppins rescues him again, and takes him back to his own time.
When Jane and Michael flee the bank through an alleyway, and encounter a creepy old lady, a large black dog, and a shadowy figure. (Fortunately, the last one is just Bert covered in soot.)
It doesn't even help that the old crone they ran into was played by Betty Lou Gerson, the voice of Cruella De Vil.
Bert's verse of "Chim-Chim-iney" where he sings about "the chimney sweep world" is very accidentally unnerving and sinister sounding. Particularly how he ends it with an almost maniacal smile after singing "on the rooftops of London...coo', what a sight!"
Just... the way Dick Van Dyke reads the line, "Where there's hardly no day, nor hardly no night, there's things half in shadow...and halfway in light". Chills, man.
The stage musical features a new, scary number called 'Temper, Temper' where Jane and Michael's toys grow bigger than they are, come to life, and put them on trial for losing their tempers and breaking a toy. Remember all the mean things you did to your toys? Cover your children's eyes... and your own.
The song was deemed so scary that it has been replaced with 'Playing the Game' in the Broadway version. It was also part of the reason the producers would not allow any child under the age of three into the theatre during its London run and stated that the show was meant for children age 7 and up.