Mentors pass on certain critical skills to their students, and at the Spandrel Shop, blogger Prof-like Substance writes that he learned from his undergrad mentor that you should love what you do. Then, as a graduate student and a postdoc, Prof-like then learned how to be a scientist and how to write, respectively. "I went to work with [graduate mentor] because of what they do, but came away from that lab knowing what it meant to be a scientist," Prof-like says. "This isn't as common as we would all like to believe."
In the comments to the post, reader Jacquelyn Gill shares that her undergrad mentor "taught me to ask -why-, and not to stop." And Scicurious added her grad school mentor taught her "to shoot for the stars. You never know you'll get rejected until you do, and if you don't apply, you'll never get in."
Others, though, were not as lucky. "Most important thing I learned, what not to do. Seriously," added BrknScience in the comments.