When I check with google index "site:http://example.com" I see a few webpages that have the real page followed by //RK=0. I have no idea what this means, how it got there, and why Google indexed it.
As a developer, I need to know why anyone would put such a suffix on the end of a URL, because it's nothing I coded for.
Someone found where this mess is coming from. http://xenforo.com/community/threads/server-logs-with-rk-0-rs-2-i-now-know-what-these-are.73853/
It looks like actually NOT malicious, it's something broken with Yahoo rewrites that create URLs that point to pages that don't exist. The demo described on xenforo does replicate it, and the pattern of the URLS that Yahoo is producing:
http://r.search.yahoo.com/_ylt=A0SO810GVXBTMyYAHoxLBQx./RV=2/RE=1399899526/RO=10/RU=http%3a%2f%2fkidshealth.org%2fkid%2fhtbw%2f/RK=0/RS=y2aW.Onf1Hs6RISRJ9Hye6gXvow-Sure does look like the RV=, RE=, RU=, RK=, RS= values are of the same family. It's just that somewhere the arg concatenation is screwing up on their side.
Maybe at one time there was a bug or something odd going on with Yahoo. It caused Google to enter these extra URLs into its index. I am going to remove them with the WebmasterTools "disavow" feature, as Google may be seeing them as duplicate content and thus causing contributing to a "Panda" penalty.