A suite that passed reliably on laptops moved to a Grid and became unreliable. No test changed. The team suspected the Grid was broken.
The Grid was fine. Distributed execution changes four things, and each breaks a different assumption.
One: latency multiplies.
Every command is an HTTP round trip, as the protocol lecture explained. Locally that is a millisecond. To a Grid in another data centre it might be 30 to 50ms.
A test making 100 commands goes from 0.1 seconds of protocol overhead to 5 seconds. A suite of 2,000 such tests adds hours.