A team enabled parallel execution and their suite started failing randomly. Different tests each run. Sometimes a click landed on the wrong page. Sometimes a test asserted on another test's data. Nothing reproduced in isolation.
They had one driver variable shared across threads. This is the most-asked senior Selenium interview question, and it is worth being able to explain precisely rather than just naming the fix.
Why sharing a driver breaks.
A WebDriver session is one conversation with one browser. Commands carry a session ID and are processed in the order they arrive.