Why do tests pass individually but fail when run together?

Tests can be run separately but fail when run together due to:

• Dependencies between tests: one test changes the state that another depends on.
• Shared environment state: if tests are not isolated, they can affect each other through the database, files, cache.
• Parallel execution: data races and conflicts during concurrent access.
• Incorrect cleanup after tests: residual data affecting subsequent tests.

To avoid this, you should:

• Make tests independent and isolated.
• Use mock objects or test databases.
• Clear the state after each test.
• If necessary, run tests sequentially or with isolation in mind.