Apparently framing is not allowed. I presume this means loading stackexchange in an iframe. However I recently started seeing that error message from stackexchange when I closed a security.stackexchange tab and then re-opened it using google chrome (they have a specific action for "reopen last closed tab"). The strange part is that I only see this happen on security.stackexchange.com. It doesn't happen on stackoverflow.com. It happens both on question pages and the security.stackexchange.com homepage.
At first I was going to dismiss it as a browser quirk (I'm on a slightly old version of chrome), but given that it only happens on security.stackexchange.com, I'm guessing it's not my browser (or not just my browser).
If this is a bug, it obviously this isn't the highest priority, but I thought I would mention it.