A seminal verse in Job reads, “Though He slay me, yet I will trust in Him (lo ayahel)” (13:15).
Traditionally, the word lo is read as if it was written as “lamed-vav,”meaning “in Him.”
However, the Masoretic text actually spells the word lo “lamed-aleph,” meaning “not,” which turns the meaning of the entire verse on its head:
“When He slays me, I will not hope.”
At first glance, the difference between the ketiv (written version) and the kerei (read version) seems trivial – it is the difference between a vav and an aleph.
But it is actually a fundamental difference, because it determines the way in which a person approaches events and how he responds to them.
–Rabbi Adin Steinsaltz