Hey guys, there's a new feature out in browsers. It's called tabbed browsing. Oh? It's been around for a while? Then why are you still developing like it doesn't exist?
Traditionally, users browsed the web one page at a time due to the restraints of browsers and their connection. These days, power users will open up multiple tabs, often from the same website, allowing them to explore a greater amount of the web than they could by clicking through one page at a time.
Often, though, it seems like web developers are unaware of this fact. Flash sites are right out, since Flash is not controlled by the browser, as are Flash menus. You shouldn't have a menu made in Flash anyway, it's a stupid idea, and Flash sites should only be employed when DHTML cannot achieve the same effect.
Further, cookies and sessions can be confused by tabbed browsing. I've encountered quite a lot of dynamic breadcrumb setups on the web that will allow something like "Home > Products > Contact" because the user is browsing the site in multiple tabs.