Back in the day I would log onto yahoo chess and beat everyone by simultaneously playing expert mode in windows chess and just mimicking the expert modes moves on yahoo as mine. Good times, never lost.
No seriously
Unless people intentionally griefed by creating new low-rating accounts to cheat at.
It's enough of a thing that there was drama and accusations of cheating/plagiarism a few years back http://en.wikipedia.org/wiki/Rybka#WCCC_disqualification_and...
http://www.husvankempen.de/nunn/ http://www.computerchess.org.uk/ccrl/4040/ http://ssdf.bosjo.net/list.htm
There are more but these are the reliable and longest running that I know of.