Normally it means two things: You deleted your website from CJ. When you generate a link it goes under a URL you once submitted to CJ. When you delete it all the links you generated on CJ under that URL become invalid. Two: You need to update your links. CJ from time to time updated the advertisers links which means you have to copy the new links and replace the old ones with them (it is very annoying but they keep doing it year after year). Just find out which links are invalid, generate new ones and place them instead of the old ones.
I had this too. In my case the click was from an expired advertiser. Although I removed the link from my page, it was clicked a few times. I assumed it was due to the browser caching the page.
some links are promotional type they expire after perticular date (discount/offer of month ads) so you have to replace that with new one otherwise they become invalid.
Actually it can mean one more thing. If you still have links up of an old expired advertiser (this is the #1 reason) then it will generate invalid clicks. Invalid clicks isn't anything to worry about unless you enjoy losing traffic to links that aren't working correctly or going to old expired advertisers. Check your expired advertiser list first and make sure.