Unit tests will not catch all issues, the person that writes the unit test will probably not see a corner case when implementing the code so it won't add the test for that corner case in the unit test either.
Or if the same person is writing the test as the code it is testing, given this is a syntax error rather than a logic error, it is likely that the test will be flawed similarly.
A test passing just means that the code and the test agree, not necessarily that both are correct.
