Facebook. The other day, Jeff and I noticed their footer bar – let’s call it the presence bar, since that is how they’ve identified the div – behaves differently with different browsers!

In Safari, Opera, and Chrome it works normally. You click a link, the page reloads, and the presence bar opens to where you’d left it.

In IE (6 and 7), as well as in Firefox, not so much. The links only alter the part of the URL succeeding the hash mark. The page never reloads.
New content is loaded via an invisible iframe. On completion, the data is extracted from the iframe and inserted into the main section of the page – which doesn’t include the presence bar.

Facebook can use JS to read the section of the URL after the # and load the appropriate page, which allows pasted URLs and bookmarks to work, as well as the back and forward buttons.
If you inspect their HTML, you will notice that the links’ destinations are identical for either set. They probably use Javascript to hijack clicks via eventlisteners.

I have no idea why they’ve decided facebook should behave differently based on browser. Maybe they ran into some sort of problem? Ideas?

I prefer the first approach, but they could have done much worse with the second. They’ve made sure it preserves usage assumptions common to web applications, and degrades gracefully.