In-house, we have confirmed a serious problem with Safari 1.2.1 and FedEx's Web-based package tracking mechanism.
You can reproduce the issue by clicking here, but be warned: Safari will stall and processor usage may spike to extremely high levels, activating the fans on your desktop Mac or PowerBook.
In our case, the issue simply caused a Safari-specific freeze, which allowed us to force-quit Safari after a few attempts. Some users, however, report that upon attempting the above URL, Mac OS X is no longer responsive and a system restart is required.
Neither Microsoft's Internet Explorer nor the latest version of Mozilla displayed this issue in-house, instantly resolving the problematic URL.