Something like this happened to me today. I noticed a floating menu that worked fine in Firefox and Safari didn't work in (wait for it) IE. After about three hours, I discovered that the JavaScript for the menu needed a CSS top attribute to work with; FF/Safari set it to 0 by default, but IE apparently needs an explicit value. Three hours, one line:

  #navtool {
    top: 0;
  }

But it works, and that's what really matters.