Search for shortcuts
There is no shortcut to GSoC selection and there is no shortcut to GSoC completion.
The student stipend is only paid after the contestans submit a minimum amount of code. Even if you do find a certain shortcut, you will not recieve your stipend without presenting meaningfulcode.
If you are the right candidate for GSoC, you will not need a shortcut anyway, as you will love completing your task.
Disappear
If your mentor thinks you have disappeared during the summer, this tends to quickly result in failure. The most common problem is failing to mention long family vacations or class schedules in the summer. Disappearing includes taking the initial payment and running with it; if you're tempted to do this, you might want to consider the damage to your reputation, or the excited students missing out on a slot so you can waste yours.
Take another major commitment
The GsoC is a full-time job, and anything that takes more than 5 hours a wekk will cause problems. Having two simultaneous full-time jobs is unrealistic and is not expected of you.
Submit code at the last minute
Showing your code in public can be scary. Some students wait until the very last minute to show their code to their mentor and other contributors to the project. Do this only if you have a burning desire to fail, because it's too late for any review to fix holes in your code.
Be shy about asking for help
This is a learning experience, and there is no such thing as a stupid question. You will face issues and problems that need to be solved, but you will need help. The mentors are here to provide feedback as well as help you when you're facing an issue, so never hesitate.
Consider this is a solo project
This is not college or highschool, Google summer of Code heavily implies working with a community. The mentor will provide feedback and help you with your issues, while the rest of the community will keep contributing to the code, making the software a living entity that grows and evolves.
Commit multiple non-related changes at once
If something is bad about one of the changes and someone needs to roll it back, it's more difficult to do so. Make each commits succinct and functional, even if it means a little extra work.
Go for more than a week without communicating with your mentor
The project timeline doesn't allow for unplanned gaps in communication. Students should talk to their mentor at least once a week to update them on their status whether the mentor asks for it or not. That way, both of them can plan ahead and keep a steady work flow.
Start coding before planning
You can encounter numerous issues if you decide to start coding before discussing the project on an architectural level with your mentor. Changes in the plan will most likely translate into rewriting a bunch of code for no reason.