Many a times it happens that if I change a pretty-permalink to one of the key pages on my site, all the other posts referencing it lead the visitors to 404 pages. That’s not good for the visitors or the SEO or even AdSense (since you are not allowed to show AdSense on 404 pages).
Xenu comes to the rescue. Xenu’s Link Sleuth (TM) will check all links to other posts, pages, images, even files in CSS external or internal. At the end it will generate a report and you can use it to fix the broken links and other issues that you may find. May be the visitors were just getting a 404 page instead of seeing the donation page. Xenu Link Sleuth is a tiny download and you can get it from Xenu’s Links Sleuth’s homepage.