Or a third rail pot blows up in Charing Cross. See other topic.
That actually went relatively well, at Holland Park last year people climbed out between the carriages. Of course, if you've told people terrorists are coming to shoot them and they should run, then there is a loud bang....
Charing Cross (the bit where it happened, at least) is above ground and has a decent number of exits. Underground, with one set of escalators and some emergency stairs, much easier for it to all go wrong.