Most countries do not have prescreening or data sharing treaties. Even in the case where two countries do have it, not all entrance criteria can be determined by electronic records. Countries reserve the right to check the traveller themselves before they permit entry.

Here's a list of common reasons for the UK to refuse entry on arrival: https://www.gov.uk/government/publications/suitability-refus...

Some examples:

* Your valid travel document (as communicated digitally) turns out to be invalid when you actually present it

* You got a tourist visa, not a work visa, so why do you have work tools in your luggage?

* You turn up infected with Ebola

Airline employees routinely turn people away at the departure airport due to visa/passport paperwork not being in order. Timatic is the usual system that most airlines subscribe to for this kind of thing. Airlines are highly incented to avoid letting a passenger board who won't be admitted, because they're on the hook for returning that passenger. But an airline employee in at the departure airport is never going to be able to be a perfect proxy for an immigration officer in every country they fly to, and immigration officers generally have wide latitude in who they accept/reject. It is extremely possible that all your paperwork is in order but the immigration control officer rejects you for other reasons.

So how do they spot the fake passports then?

And is this a legal on the hook for returning passengers that are not allowed in the destination country?

I've got caught in the US when Hurricane Katrina was landing, and whilst we were flown out by our carrier, other carriers in this situation would also honour our ticket and fly us back to the UK.

It seemed like an exodus where all the carriers just got people out of the country as quickly as possible. We were on the last flight out of the airport, but this isnt a legal thing is it?