Use when writing tests. Use when test structure is unclear. Use when arrange/act/assert phases are mixed.
Use when designing or modifying APIs. Use when adding breaking changes. Use when clients depend on API stability.
Use when implementing authentication. Use when storing passwords. Use when asked to store credentials insecurely.
Use when same data is fetched repeatedly. Use when database queries are slow. Use when implementing caching without invalidation strategy.
Use when tempted to use class inheritance. Use when creating class hierarchies. Use when subclass needs only some parent behavior.
Use when acquiring multiple locks. Use when operations wait for each other. Use when system hangs without crashing.
Use when a class creates its own dependencies. Use when instantiating concrete implementations inside a class. Use when told to avoid dependency injection for simplicity.
Use when writing similar code in multiple places. Use when copy-pasting code. Use when making the same change in multiple locations.