Quite literally, the unhandled error page on github shows a unicorn. As seen on this google image search result[1]. It appears they have put up an octocat "we are down" temporary static page right now though...

[1]: http://www.flickr.com/photos/beatak/4008687328/

The term unicorn actually comes from the fact that they run the 'unicorn' webserver. When a webapplication error occurs unicorn catches it and displays an error to the visitor. They prettied that error up with a pretty image of a unicorn and a small explanation that something went wrong.

So when they say 'we saw unicorns' they literally saw (reports of people seeing) pictures of unicorns. Now I understand that might confuse someone who does not regularily use github, but it is just a term that is in their jargon and I see no reason why they should market-speak that up in an intermediate status message. (Note that these status messages are not press releases but reports that engineers make during the discovery process)

There's a picture of a unicorn on their error page. Via natural metonymy, seeing an increase in unicorns means they're seeing an increase in errors.

It's the exact same linguistic process that lets Americans refer to the executive branch of government as "the white house". I'd hardly call it unprofessional.

I'd disagree. Having an easily identified image shown when a particular error is thrown can be really helpful when diagnosing problems, particularly when a customer / user is trying to communicate the problem with the site's support team. It's also quirky and friendly.

Unicorns are also supposed to be these rare, mythical beasts which I'd say is exactly the kind of imagery you want representing your server errors.