The other day I received an email from a grad student that read as follows:
I'm a second year PhD track grad student, and my advisor is in his 3rd year of a TT position. As a request: I'd really like to hear your thoughts on what it's like to mentor PhD students for the first time. PhD students are an odd bunch, and It would be really nice to get a sense of what my TT prof is going through. I once heard a tenured prof talk about her first grad student, who dropped out after two years. She called him her "burnt pancake", because 'the first one never turns out right'. I don't really wanna be that pancake.
Fair enough, no one wants to be the guinea pig as someone else tries to figure out how all this works. At the same time, if the tenured prof from the email has (presumably) successfully mentored other students since (meaning that she isn't a tyrant or obstructionist), I would guess that it has far less to do with the advisor than it does the student. She may have been rationalizing the failure of her first student, but as the supervisor of a non-zero number of grad students I have found one thing very clearly: they are all completely different.
What I mean by this is simply that the process of mentoring a student who is interested in the work they are doing and intellectually up to the challenge is not all that hard. Yes, you learn a few things along the way that might head off some issues, but overall our job is to keep students on a productive path, get them resources and be a sounding board for ideas, problems and progress. Sticking to those points seems to work pretty damn well for most students.
I think more at issue here is the what the PI sees as failure is not necessarily what the students sees as the same thing. If a student leaves the lab (perceived as "failure" in most academic circles), it could be as much to do with the fact that they decided the life wasn't for them than anything else. They may have had a different opportunity or decided that they didn't like the path ahead. Maybe they had a long distance relationship that they didn't want to be long distance anymore. Who knows? But there is a good chance that they say leaving the lab as more of an opportunity than a failure, whereas the PI decided that she needed a do over because if she had more experience she could have kept that student interested in the amazing research she was doing. Obviously I don't know any of the back story, but I'm guessing that the student found something that was better for their life and the supervisor, unable to picture that possibility, decided that she must have messed up that student because she wasn't warmed up just yet.
I guess my advice to students of junior faculty is to focus on your project and pushing it forward. If your PI gives you the resources (physical and intellectual) to do something you enjoy, while still be available to help you when you need it, then I wouldn't worry too much about turning out like the first pancake out of the pan. If you like what you are doing and have a functional relationship with the boss*, there are not many ways in which "mentoring" can go horribly wrong just because the PI is supervising their first student. In fact, there are many ways in which being one of the first students in the lab can be a good thing, as long as your PI doesn't get crushed by the weight of getting funding (which can happen at any career stage, FWIW). Use your network of friends and your committee members if you think you need more perspectives than just that of your PI and be a little selfish about advancing your own career.
And if that means leaving a lab for a different opportunity, know that your move will only be seen as "failure" by the people you are leaving behind.
*Obviously, if your PI turns out to be a total douchecanoe, this is a whole other problem.