Since I signed up on GitHub I was searching for this bug: to "draw" things on others contributions graphs. And finally I found a trick. I cannot tell you exactly how it works, but the docs are very clear, you will get it: https://help.github.com/articles/why-are-my-contributions-no...
I disclosed this trick to the security team, but they said it's a low priority bug and I cannot get a reward for it (https://bounty.github.com/).
So, I thought: if I cannot get money for this discover, I will get some popularity. And I "vandalized" some contributions graphs:
Then I shown the GitHub Security team the mess I did. They told me not to do that again since this is a violation of the GitHub's terms. I kept that in mind and didn't repeat it.