Keanu Reeves kept it coming in The Matrix movies, but the role of Neo wasn't initially offered to the actor. Will Smith turned down the role saying, "I would have absolutely messed up The Matrix. At that point I wasn't smart enough as an actor to let the movie be — whereas Keanu was." It takes a mix of maturity, self-awareness, and instinct to know when a project is more than you can handle, and it's an important lesson for our working lives.
Will's career was obviously unaffected by his decision to reject the role; in fact, if he went forward against his instincts, perhaps he wouldn't be flashing that smile as the biggest-earning actor in Hollywood. We should always be up for new challenges that add to our career development and take calculated risks, but know when it's better to back away from a project that's over your head or if you aren't nearly qualified for a job that is offered to you. Be confident about your abilities, but be realistic and let your instincts guide you.