Under what conditions can a mock object serve a bad purpose?

Mock objects can be detrimental in the following cases:

• Overly strict expectations: if a mock is set up for very specific calls with exact parameters, tests become fragile and break with minor code changes.
• Disconnection from real behavior: a mock may not reflect actual logic or side effects, leading to a false sense of correctness.
• Masking problems: using mocks can hide integration errors or issues with external dependencies.
• Maintenance complexity: a large number of mocks complicates understanding and maintaining tests.

For example, if a database mock always returns fixed data, the test will not detect errors related to real data changes or database behavior.