HTTPS Everywhere is pushing the Heroku page to HTTPS, which breaks because while you've got a server listening on port 443, it's presenting a Lanyrd certificate.
Definitely an issue with my configuration, but might well affect a few of your more security concious visitors.
I have HTTPS Everywhere and I'm shocked at the number of sites I've found that don't work like they should when it's enabled. Namely a certain stock charting website that uses javascript to force you to HTTP unless you are a pro member that gets HTTPS Everywhere stuck in a loop fighting with it.
http://events.heroku.com/
http://facebook-developers.lanyrd.com/
http://github.lanyrd.com/
http://lanyrd.com/mashery/
http://lanyrd.com/happy-cog/