That's a result of the Slack badge at the bottom of the page trying to use a WebSocket to update its counts. We discovered getting WebSockets working through ELB was not trivial and have decided instead to disable the real-time updates to the badge. The changes have been made for that, but sadly, not deployed in time for your visit.

Anyway, the errors in the console aren't impacting the behavior of the site (the badge is falling back to polling to get updates, for instance). They are ugly, though, and will be gone in a future deployment.