Passengers are turned away at check-in more often than at borders. The reason is that airlines carry financial liability for anyone they bring who is then refused entry.
Carrier liability drives the check
Where a passenger is refused entry, the airline is generally required to return them and may face a penalty on top of the cost of the flight.
That exposure gives carriers a direct incentive to check documents thoroughly before departure rather than rely on the destination to sort it out.
The check is consequently conservative. Staff refuse where there is doubt, because the cost of a wrong refusal falls on the passenger while a wrong acceptance falls on the airline.
Passport validity rules are the most common failure
Many countries require a passport to remain valid for a period beyond the intended departure date, commonly several months, and the requirement varies by destination.
A passport that has not expired can therefore still be unusable for a particular trip, which is easy to miss when checking only the printed expiry date.
Blank page requirements catch people similarly, since some borders need physical space for a stamp or visa and will refuse entry without it.
Onward travel evidence is checked more than expected
Where entry is granted for a limited period, many countries require proof of departure, and airlines verify this because they inherit the liability if it is missing.
A one-way ticket into such a country is frequently refused at check-in regardless of the traveller's intentions or resources.
The requirement is applied unevenly between airlines and airports, which is why experiences differ so widely for the same route and nationality.
Name mismatches are treated seriously
The name on the ticket must correspond to the travel document, and differences arising from marriage, transliteration or middle names can cause problems.
Systems match on limited fields, so an abbreviation or reversed order can trigger a check even where the passenger is clearly the same person.
Correcting a name after booking usually involves a fee and a lead time, so it is worth verifying immediately after purchase rather than at the airport.
Requirements change with limited notice
Entry rules are set by the destination and can alter between booking and departure, sometimes taking effect on a date that falls mid-trip.
Because the details vary by nationality, residence and routing, and change over time, the only reliable source is the destination's own official guidance shortly before travel.
Checking again a few days before departure is a small step that addresses the failure mode most likely to end a trip before it starts.