Ahh. If this happened only once, then I think I know what caused it.
At the top of the hour, every hour, a backend server is scheduled to do a quick (<10 second) reboot. You just happened to be trying to load a page when that server was rebooting, so the page looked partly broken to you. If it happens again, check the time and see if the time is XX:00 or XX:01.
(It's an incredibly annoying problem sometimes, but it's always gone quickly and we have no way of not making that server reboot, so it's a problem we've just had to deal with)