Response from Google ticket #55737894:

–

Hi Matt,

Thank you for taking the time to report this problem. We are aware of this
bug, and our Toolbar engineering team is working to find a solution.

We apologize for any inconvenience and hope to resolve this issue in the
near future.

Regards,
The Google Team

Both issues pertain to problems arising when the Subscribe feature of the Google Toolbar is enabled

Relevant prefetching recommendation from Mozilla:

http://developer.mozilla.org/en/docs/Link_prefetching_FAQ#As_a_server_admin.2C_can_I_distinguish_prefetch_requests_from_normal_requests.3F

This is actually a duplicate of CONF-6803. That issue describes the problem better, as well as a workaround.