Percentage signs causing errors

Hi, I have discovered a problem with the contents of my pages. I have found on a number of pages (and I think a number of older versions of wordpress) that if the main body of text contains a percentage sign then when I go to publish or preview the page I get a error404 nothing found error. What is going on and how do I fix it?

This is a problem that has only surfaced recently and pages that had been working fine no longer do if they contain a percentage sign.