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If you search for a keyphrase and hover over the links, the web address of the site shows in the status bar. Click on that link, let that page load then click "back" in your browser. When I do that and hover over the link again it has changed to a Google encoded/tracking URL whilst all the other SERP links remain the same. Click on a second link, click back and that link has changed to a tracking link too.
This only happens to me when I'm logged in.