How do you troubleshoot flaky Cucumber scenarios caused by shared mutable state across step definition classes?

Flakiness that appears only sometimes, especially under parallel execution, and often involves one scenario's data or assumptions unexpectedly showing up in an unrelated scenario, is a strong signal of shared mutable state leaking across what should be isolated scenario executions.

1. Check for static or singleton fields - any static field, or singleton pattern, in a step definition class or a class it depends on bypasses Cucumber's normal per-scenario instantiation and persists across scenarios (and across threads, under parallel execution), which is the single most common root cause.
2. Confirm dependency injection is actually being used correctly - shared state should flow through a properly DI-managed context object scoped per scenario, not through manually created singletons or static holders that accidentally bypass that scoping.
3. Reproduce with a minimal, isolated pair of scenarios - run just two suspected scenarios together, with and without parallel execution enabled, to confirm whether the flakiness is specifically tied to shared state versus some unrelated timing issue.
4. Check for shared external resources without proper isolation - a shared test database, file, or in-memory cache used across scenarios without per-scenario cleanup or unique keys can produce the same symptom even with fully correct DI-scoped Java state.
5. Disable parallel execution temporarily as a diagnostic step - if flakiness disappears entirely when forced back to sequential execution, that strongly confirms a concurrency/shared-state issue rather than a genuinely non-deterministic application behavior.

The underlying principle is that Cucumber's per-scenario instantiation and DI scoping only provide isolation for state that actually flows through that mechanism; any state that escapes it, via static fields, external shared resources, or manually managed singletons, silently reintroduces the exact cross-scenario coupling the framework's design is meant to prevent.