The other day I was horrified to receive an email from Helium accusing me of plagiarism because an article I had posted there had been found on another site with a different byline. When I googled a couple of paragraphs I found it on an article ezine. This one had already been purchased from CC for usage rights only. That means it can be published once with my byline and I still retain the copyright. CC are now chasing the buyer to see if that person was responsible and, if so, to request a changed byline, or an upgrade to full rights on payment of the difference in the prices.
I have explained the situation to Helium via their help team, ie, that I am a victim not a perpetrator, and await a response. The pennies I'd earn there from that article are probably not worth bothering about, but I'd really like my reputation to remain intact.
The lesson I've learned from that is to set up google alerts on all my articles from now on, so that I'll most likely find out early if they have been stolen or can check how they appear before posting them anywhere else.