You may have noticed that in the game it is not possible to travel to a city again after you have already visited it. The game engine automatically disables any routes that leads to a previously visited city. This is all good until you stumble into a city with no way out because you have already exhausted all leaving options beforehand.

Normally this wouldn't happen as you should aim to get back to London as quickly as you can, taking the most direct and obvious transport routes. But for people who like to explore as much as they can, it presents a danger to end their run prematurely as they got stuck.

This isn't such a big problem before the additional content is released. It is relatively easy to get trapped in the current version of the game. Poor route design and insufficient testing may be the cause of this. Here are a number of such death-traps I have found (minor spoilers, maybe), with the "journey of no return" in thick arrow:

The Russian trap
Istanbul -> Kabul -> (Omsk) Moscow -> St Petersburg ==> Minsk -> Odessa (dead end)
Istanbul -> Novorossiysk -> Tsaritsyn -> Moscow -> St Petersburg ==> Minsk -> Odessa (dead end)
(More variations exist for this trap)

The Egyptian trap
Suez -> Khartoum -> Jeddah -> Riyadh -> Ha'il -> Cairo ==> Aswan -> Wadi Halfa (dead end)

The Asian traps
Beijing -> Vladivostok -> Yokohama -> Manila ==> Hong Kong (dead end)
Batavia -> Manila ==> Singapore
(May be already patched by the developers)

The Caribbean traps
Port au Prince -> Washington -> Atlanta -> Miami -> Havana ==> Port Royal (dead end)
Port au Prince -> Washington -> New York -> Ottawa -> Winnipeg -> Chicago -> Atlanta -> Miami ==> Havana -> Houston -> New Orleans (dead end)
(More variations exist for this trap)

The American trap
Atlanta -> New York -> Winnipeg ==> Chicago (dead end)
Winnipeg -> New York -> Atlanta ==> Chicago (dead end)

And then there is this situation... I haven't tried it myself, but on paper it is entirely possible for a player to have visited all of Budapest, Khartoum, Bhayi and New Orleans when they reach the end of the Black Rose storyline. I wonder what would happen then if the player is unable to get on any of those routes in Dakar. Heh heh.

At any rate, I believe the developers should add an exception when players are stuck like this, allowing them to re-visit a city and get out of the mess. Unique additional dialogs (like Fogg blaming Passpartout for going in circles) would be welcome too.