At the start of Hitagi Crab
in an accident involving a misplaced banana peel, Hitagi falls down the flight of stairs that lead to the upper floors of Naoetsu Private High School and ends up in the arms of Koyomi Araragi
but the oddity that Hitagi was inflicted by was the Heavy Stone Crab which took away her weight which means she's weightless and sorta like being in space so when she tripped she wouldn't fall down like unless someone gave her a push after.
not to mention that when Araragi catches her he feels that she's weightless which is how Hitagi's "secret" was found out by him
How does Hitagi fall if she's weightless?