The malachim tell Lot and family not to look back at Sodom. Lot's wife looks back.
The pasuk tells us Bereishit 19:26
וַתַּבֵּט אִשְׁתּוֹ, מֵאַחֲרָיו; וַתְּהִי, נְצִיב מֶלַח.
"But his wife looked back from behind him, and she became a pillar of salt."
Rabbi Levi ben Gershon (Ralbag) suggests that וַתְּהִי does not refer to Lot's wife, that *she* became a pillar of salt, be rather וַתְּהִי refers to the *city*, which is a feminine noun, so Lot's wife looked back and saw that the city had become a pillar of salt, which was a way of saying it was destroyed.
Of course, we would then have to explain where Lot's wife is in the next story with Lot's daughters. Perhaps she was arrested for assault.