Entry rules are set by destinations and enforced first by carriers, which is where most travellers encounter them.
Carrier liability
Airlines fined for carrying passengers who are refused entry.
Which is why documents are checked at check-in.
Visa-free does not mean unconditional
Passport validity, onward tickets and proof of funds.
Which are frequently required and rarely mentioned.
Electronic authorisations
Pre-travel systems short of a visa.
Which several regions have introduced.
Transit
Separate rules for changing planes.
Which catch travellers on connecting itineraries.
Why airlines are strict about documents
If a passenger is refused entry, the carrier is generally fined and must return them at its own cost.
Which means check-in staff have a direct financial incentive to refuse anyone whose documents are not clearly in order.
They are applying the destination's rules on the destination's behalf, and they will err on the side of refusal because the cost of being wrong falls on them.
Passport validity rules
Many countries requiring validity beyond the intended stay.
Which catches travellers with months remaining.
Onward and return tickets
Proof of departure required at entry or check-in.
Which affects one-way travellers.
Purpose of visit
Tourist authorisation not covering work or study.
Which includes remote working in some jurisdictions.
Where to check
The destination's own immigration authority rather than third-party summaries.
The requirements people most often miss
Passport validity beyond the stay, blank pages, proof of onward travel, and authorisations that must be obtained before departure rather than on arrival.
Which are all checkable weeks ahead and are frequently discovered at the airport.
Electronic travel authorisations in particular catch people out, because they are new, they apply to previously visa-free travel, and they cannot be obtained at the border.
Dual nationality
Which passport to travel on affecting requirements.
Which some countries have specific rules about.
Previous travel history
Stamps and prior visits affecting admissibility in some cases.
Overstaying
Consequences ranging from fines to entry bans.
Which are recorded and persist.
Timing applications
Processing times varying enormously.
Which argues for starting early.
Why the travel industry is so hard to see into
Almost every part of a holiday is sold by one company, delivered by another and regulated by a third, frequently across several countries. The traveller deals with a single price and a single confirmation, and the arrangements behind it are invisible.
That structure is not designed to confuse anyone. It exists because travel involves aviation, accommodation, ground transport, insurance and consumer protection, and no single business does all of those well. The consequence is that when something goes wrong, working out who is responsible is genuinely difficult, and the answer frequently depends on how the booking was assembled rather than on what actually happened.
The questions worth asking before booking
Who is the organiser, what protection applies if a supplier fails, what exactly is included in the price, and what happens if plans change. Four questions, all answerable at the point of booking, and between them they cover most of the situations that produce complaints afterwards.
None of this makes travel more complicated than it is. It makes visible a set of decisions that are being made whether or not the traveller notices them.
A general note
Consumer protection, aviation passenger rights, classification schemes, driving rules and insurance regulation all differ substantially between countries, and change. National regulators and consumer bodies publish the applicable rules, and they are the authoritative source for anything with money or a legal position attached.
One habit worth adopting
Before paying for anything in travel, read the part of the confirmation that describes what happens if it does not go ahead. Not the terms in full, just that section.
It takes two minutes, it is where the meaningful differences between apparently similar offers actually sit, and it is the only part of the documentation that matters on the day something goes wrong.
A closing thought
Travel is one of the few substantial purchases most people make without reading anything about how it is arranged. That is understandable, because the arranging is meant to be invisible and usually is.
The occasions when it becomes visible are the occasions when something has gone wrong, and that is the worst possible moment to be learning who is responsible for what. Ten minutes of reading before booking is not a burden, and it is the difference between having a position and hoping someone will help.
Where to find reliable information
National tourist boards, aviation regulators and consumer protection bodies all publish material aimed at travellers rather than at the industry, and it is free, specific and rarely promoted. It is considerably more useful than the general advice that circulates, and it has the advantage of being current for the country you are actually going to.
A final practical note
Almost everything above rewards a small amount of preparation and punishes none. Checking entry requirements, reading what a booking includes, and knowing what protection applies takes an hour spread across the weeks before a trip.
It is the least enjoyable part of planning a holiday and it is the part that determines whether an ordinary problem stays ordinary.