My error logs were recently filled with repetions of the the following line:
[Date] [warn] [client IP] Not GET request: 2., referer:
As it turned out this was caused by the mod_pagespeed module (from Google). Disabling it has stopped the errors, I will be trying out a newer version of the module shortly.
Problematic version: mod-pagespeed-beta-0.9.0.0-128.i386
Approximate Date: Nov 2, 2010