How do you advise an early career researcher who has an unrealistic opinion of their prospects? One often reads stories along the lines of "I was told I wasn't suited to a career in academia; now I'm a professor at &lt; Big Name University >". At the same time, given the structure of the academic job market, many people who desire an academic job will never get one. I am sure many of us can think of individuals who made big sacrifices in an unsuccessful pursuit of an academic career, despite never really being a competitive candidate. Equally, we can probably also think of people who succeeded, against our prior expectations.

Given all the above, what should an advisor do/say when confronted with an early career researcher (graduating PhD student/junior postdoc) whose career goals appear incompatible with their current trajectory? How do you support and encourage your advisee, while also being realistic?
Honestly, without putting down. 

Explain the weaknesses of their approach or profile, explain what they would have to change in your opinion to get where they want and explain what they can do if it does not work this way. The point is not discouragement, but letting them understand what their options are. Some people are able to rise far above their original level, just as some never exploit their possibilities.

Whatever you do, you can not go wrong if you understand that the choice remains theirs and yours is only to show them the map.