I think the global_site_search_rewrite function of Global Site Search is rewriting URLs generated by Social plugins such as the Fixed Social Buttons plugin. That's my best theory anyway after just a glance at the code based on what is happening here:
That's quite a broad re-write rule. Maybe I'm drawing the wrong conclusion, but is there an easy way to fix the issue (other than disabling the Fixed Social Buttons plugin, which is not really a solution).
Seems like this issue would also occur in other instances when plugins or templates pass the current URL as a querstring parameter to links on the page (obviously a common occurrence when implementing social sharing links and scripts).
Many thanks for your time!