Unobtrusive javascript is a great practice to follow. Having pages that can fall back to real links and requests if javascript is unavailable is a great safeguard. The thing is, though, that we sometimes don't really expect things to be invoked that way. It is just a fallback, right? We may forget to test screens down the road that are never supposed to be used like that. You know, like that form that is always rendered through ajax into a modal window.

You are being a good web developer and using jQuery or Prototype or whatever and have all your event handlers attached in the DOM onload. What that also means is that someone on a slow internet connection might take a few seconds to load your page. During that time, if they click any of your anchor tags with hrefs that are supposed to invoke ajax calls, it will actually browse to the page since the onload hasn't been executed yet.

So, they click that link and bam! There they are at some page that was supposed to render in a modal window and now looks ridiculous as an actual page since you haven't seen it that way in a dozen iterations. Whoops!