In my case, they started after the DNS got messed up, then fixed, so I suspect that the (outdated) cached page (or cookie, or file) that my browser was using was pointing to a file or server that was no longer accessible.
Once I cleared out the cookies and the cache (1-2 weeks ago?), the 403 errors have not (yet) reoccurred.
As such, I stand by my assertion that something in the cookies or cache on my computer did not match what the server was currently expecting and as such the problem was inside my browser
Now, had the 403 errors continued after clearing the cache and the cookies on my browser, I would agree with you that it was due to a server misconfiguration rather than something wrong with what my browser was requesting.
Aaron Z