Add a ShowMe column to your board.
Bob has just completed a task. His task was to create the test case for a successful credit card transaction. He moves the task into ShowMe and finds someone on his team. In 5 minutes Bob has showed Julia (a developer on his team) his test case and made a few modifications based on feedback from Julia. The task can now move to done.
The ShowMe column promotes understanding and cross-skilling in a team. The ShowMe is for the person who completed the task. It is based on the rubber duck theory, that explaining what you have done out loud to someone (or thing) else will highlight most of the problems immediately.
Thanks to @janetgregoryca ‏for this tip.