The biggest mishires I've experienced were generally students with a whole lot of history in their GitHub accounts. I haven't done a thorough post-mortem on what was going on there, but I wouldn't rule out plagiarism or an ability to make small tweaks but little sense for dealing with a larger, more complex project.

What we found was the way you figure out whether or not someone with a strong GitHub history is going to be a good hire looks more-or-less exactly like the way you figure out whether or not someone with zero GitHub history is going to be a good hire. All focusing on GitHub really seems to get you is the ability to cast an artificially narrow net.