Who's driving the boat?
At the end of the film, Liam has killed a whole boatload of kidnappers, but the boat remains on its course, moving at quite a pace. One would imagine that any boat captain who was aware of the relentless slaughter happening on board would try to escape. Liam himself doesn't seem to want to investigate the bridge. If the boat is not being driven, it is going to crash.
The line has also been used to describe other films in which narrative holes are obvious.
The phrase was graffitied underneath a poster for the film near London Bridge station, something Mark Kermode described as "wonderful".
The line was later adapted by Stephen Fry into "Who's driving the mandolin?", in reference to a scene in Hannah Montana: The Movie (starring Smiley Virus) in which said stringed instrument appears to be accompanying Virus, who is singing a song and playing an acoustic guitar to her father. So where is the mandolin coming from? Is someone behind the fridge? Who's driving the mandolin?
The question of "Who's driving the boat?" was answered on the show by Taken writer Luc Besson in an interview with Edith Bowman. Besson confirmed that the boat driver was listening to music through earphones and was therefore understandably oblivious to the repeated bursts of automatic gunfire in close proximity. This answer, however, fails to address Kermode's sub-question: "If it's somebody (driving the boat), don't you need to go and kill them?"