Integrating Playwright with Flask creates a fundamental clash between synchronous WSGI servers and asynchronous browser automation. This conflict manifests as the infamous RuntimeError: This event loop is already running. The root cause is architectural: Flask's blocking model collides with Playwright's native asyncio design, which manages WebSocket connections to browser binaries via the Chrome DevTools Protocol.

Common workarounds like `nest_asyncio` are dangerous for production. They patch the event loop but can cause deadlocks with Playwright's WebSocket protocol, leading to non-deterministic failures. Similarly, using Playwright's `sync_api` with `gevent` workers is incompatible due to greenlet conflicts and monkey-patching collisions. The error is not a simple bug but a symptom of deeper concurrency mismatch.

For production stability, three architectural patterns are recommended. The safest is offloading tasks to a Celery worker queue (Pattern B), decoupling web servers from resource-heavy browser processes. Alternatively, a threaded dispatcher (Pattern A) isolates the event loop in a background thread. For new projects, migrating to an ASGI framework like FastAPI or Quart (Pattern C) aligns the server with Playwright's async nature, eliminating the conflict entirely.