How NOT to be selected for GSoC
While there's no guaranteed way to be accepted into GSoC, for starters because organizations themselves don't know how many slots they will be allocated by Google and because some projects are very competitive and more often than not very good proposals needs to be rejected, there's definitely some actions that will get you discarded very quickly.
If you really want to participate in GSoC and not just apply for the sake of applying, avoid this:
Writing an email that screams that you are sending it to many orgs at the same time. We can see through this - things like not mentioning the organization by name or being totally generic in the content, without specific questions, are dead give aways.
Sending your resume and asking the organization to find a project for you.
Asking questions that can be found in either GSoC's website or the org's website. We are happy to answer questions, but if you reach out to ask how GSoC works, or the timeline, etc, it shows that you haven't spent five minutes doing the most basic homework.
Being pushy with mentors, pinging non stop. Of course you can reach out and send a reminder from time to time in case the mentor didn't see your last communication or forgot to reply. But if you ping every 3 hours, that's not going to work. Why? Because mentors also have personal lives, GSoC is just one part of it, and if you can't wait a bit during the application period, how is the collaboration going to be when the summer comes?
Sending last minute proposals without any previous communication with the community. Even if the proposal is incredible, if we haven't interacted with you, we don't know if you wrote it, or if you will be able to deliver.
Of course, vague proposals that just promise to work hard without any specifics on what you plan to do.
Emails with the content "help me write my proposal".
Sending proposals that in general show that you haven't taken a look at the rules of GSoC. This takes many forms, from doing the work between several people to ignoring deadlines.
Proposals with a lot of filler / padding / etc. A 10 page proposal with 8 pages of generic things is a bad proposal.
Proposals sent to the wrong org. We've seen this happen. Double check! If we get a proposal for a different org we're not going to reach out to you, we will just throw it to the trash. Sorry. Google keeps orgs to a high standard when it comes to applying (we put a lot of work) and we pass that standard down to contributors.