Repro steps:
(0) Save a credential for www.barrysbootcamp.com.
(1) Load http://www.barrysbootcamp.com/reserve/index.cfm?action=Account.login in the simulator.
(2) Inspect the tab in Safari Developer tools.
(3) Attempt to get the username filled.
(4) Check the console logs in the Developer tools window.

The console shows: "Blocked a frame with origin "http://www.barrysbootcamp.com" from accessing a frame with origin "http://staticxx.facebook.com". The frame being accessed set "document.domain" to "facebook.com", but the frame requesting access did not. Both must set "document.domain" to the same value to allow access."

It points to the final part of __gCrWeb.fillPasswordFormWithData of password_controller.js, which does:
    // Recursively invoke for all frames/iframes.
    var frames = win.frames;
    for (var i = 0; i < frames.length; i++) {
      if (__gCrWeb.fillPasswordFormWithData(
              formData, username, password, frames[i], opt_normalizedOrigin)) {
        filled = true;
      }
    }

The filling does not succeed, and fillPasswordFormWithData returns false. The user-visible result is: autofill not working.

When that part of the code is commented out, the same error is triggered by extractFormsAndFormElements_ of autofill_controller_.js.