Several posts talk about things being not spotted by the pilot's walk-around.
Wishful thinking!
The reality is that the walk-around happens typically at least twenty minutes before the doors close - there are one or two other things to achieve in the flight deck as well, after all. Twenty minutes prior to doors closing there are many things still going on: cargo doors are open, engineers are still doing their own tasks, refuelling is under way and so on. It's not possible in the real world to actually check many of the things that can and are omitted at that stage. The only way a walk-around could be said to be realistically useful is to do it when everything has been completed and only the one door is left open - the one the walkee will use to get back in.
Like so many other parts of the operation, many things have to be on trust in the real, as opposed to the legal, world.