This was actually the idea behind using ctrl+alt+del for login. Originally MS wanted a dedicated key for this, but IBM declined (or so the story goes), so they settled for ctrl+alt+del instead.
simple.com kinda-sorta had this idea, but without the denying transactions bit (and I kinda want the ability to set a goal that just gets money added to it every month, indefinitely, but that's not a feature :( )
if the net connection dies while page resources (but not the actual html) are still loading, the already-loaded-partial-page is replaced with an error message