There was at least 1 really large error that slipped into production because we were mocking an untyped service incorrectly (a mistyped property name), and the test passed, but prod exploded. It was embarrassing, and I certainly would have caught it with more mypy discipline, but overall the pros outweighed the cons.

At big companies that use mypy, however, this is problematic. If teams only define types for the straightforward parts of the system, then the types become less useful, especially as the company scales. From what I’ve seen, individual teams use an all-or-nothing approach with MyPy, so it’s unusual for an individual dev to sparsely type anything.