As a team lead I was completely surprised by the consequences of this. Most of the time when you indulge a junior engineer trying to stretch their wings they end up failing as you might expect. As long as they aren't going to throw a sprint completely out of whack, cause massive deadline slips or damage critical code modules I usually let them have at it, often with some boundary on the time I'm willing to allow them to make the attempt. My experience is that preventing someone from "scratching an itch" leads to them losing motivation very quickly. But the surprises are the successes. I've seen junior engineers pull off some extraordinary efforts resulting in objective improvements that positively affect the entire team. Those moments are among the most rewarding and memorable of leadership. I thought they were going to learn a lesson but they end up teaching me one about trust.