The flake that only appeared under parallelism

A test that had passed ten thousand times serially and failed roughly one run in twelve once eight processes shared a clock.

// the test
$order = Order::factory()->create(['placed_at' => now()]);

$this->travelTo(now()->addSeconds(30));

self::assertTrue($order->isWithinCancellationWindow());

// the window is 30 seconds, inclusive at one boundary.
// under load, `now()` at creation and `now()` in the
// assertion could differ by enough to cross it.

Serial execution made the timing deterministic by accident, and parallelism removed the accident rather than introducing a bug. The fix is an injected clock rather than a longer window — a test that depends on wall-clock elapsed time is a test with a race in it, and the race was always there.