As a GSoC mentor, you are in the best position to identify "red flags" and warning signs early on. This is critical to addressing problems before they get out of hand. Learn from the mistakes of others, and learn how to identify and mitigate the following warning signs.
Not enough hours in the day: If your student has a full-time job or is attempting to defend a graduate thesis during the summer, that is probably going to not work. Even though your student thinks they will have enough extra time, don't believe them. They won't. If your student has every single minute of every day completely booked, any unexpected event, such as getting sick or a family emergency, derails this plan beyond repair. If your student cannot commit to a specified time schedule, this is an immediate red flag that they need serious help with time management.
Missing student: Missing a predefined meeting is a serious warning sign that your student is not taking the process seriously and this should be remedied as early as possible. If your student was in such a deep "coding zone" that they forget the meeting, and you have it later on, that might be acceptable.
"My village was invaded by aliens": What is a valid excuse? Students have been known to come up with outlandish excuses as to why they are not meeting their milestones. (You did agree on milestones beforehand, right?) If you think that your student is lying to you, that is a huge warning that things are going sour. Make sure to remind them that "real life just got in the way, I will redouble my efforts next week" is always better than "my village was invaded by aliens."
Bad students happen to good mentors: One thing to keep in mind: Sometimes bad students happen to good mentors. Don't take it personally. If a mentor tries their hardest and their student fails, this does not reflect badly on the mentor.
"Actions Speak Louder Than Words"
This bit of folk wisdom is proven year after year during GSoC. A mentor will write to the mentors mailing list asking for advice about a student that has not performed or gone missing for a period of time. Mid-term evaluations are coming up and the student has reappeared, reinvigorated, with no shortage of excuses. Now the mentor must decide: "Should I pass this student?".
The GSoC mailing list contains story after story from other mentors who ignored the warning signs, only to be disappointed later. A mentor considers this advice, but focuses on the unique aspects of their particular situation and passes the student...and their story is added to the list the following year.
Here are some specific scenarios to watch out for:
The Disappearing Student: A student submits a great proposal and is enthusiastic about discussing the project and getting started. You rank them high, they get accepted, and then they drop off the face of the earth. Fail.
Underperformance: The student sticks around for the community bonding period, but then it comes time to work. When it comes to actually writing code, they seriously under perform. They offer excuses when pressed, and offer scraps of code here and there. Then comes the mid-term evaluation and start committing like crazy. Fail.
Wrong Priorities: The student passes the mid-term and then goes on an unscheduled holiday for two weeks or starts a part-time job and the quality or quantity of work is seriously affected. You discuss this with them immediately and they promise to reprioritize, but the work is not produced. Fail.
Depending on your personality type, some of these might seem harsh. You might also shoulder some of the blame because you think that if you had been a better mentor, more on top of the situation, it would have been avoided. But even if you are partly to blame, so is the student. And it is up to the student to perform when expectations are communicated and agreed upon.
Pro Tip: In the event your student's boat is sinking, all mentors are equipped with emergency rocket-propelled jet skis.