Sometimes you put in the right link, but without knowing that the link has become invalid (probably due to change in URL or the page has been remove). This always happens when you insert images from other sites and then later the image doesn't appear anymore. I've seen a lot of it in forum. LinkChecker will check any web page for any broken link. Just enter the address and click 'Check'. If you have many links, this website will do all the link checking work for you. Here is the site.