Visa rules are dull, they change often, and they are enforced by people with no discretion. Almost every problem is one of five things, and all five are avoidable with an hour of preparation.
Nothing here replaces checking the current requirement for your passport at your specific entry point, on the destination government’s own site.
The five that go wrong
Passport validity. Most countries want six months beyond your entry date. Airlines check at the gate.
Blank pages. Several countries require one to four completely blank pages. Stamped pages do not count, and adding pages is no longer possible in most countries.
Onward ticket. Widely required, and enforced by airlines at check-in even where immigration would not ask.
Applying from the wrong place. Some visas can only be obtained from your country of residence, which is a serious problem in month four of a trip.
Miscounting a rolling window. Overstaying is an immigration offence, not an administrative slip.
The types
Visa-free · Travel authorisation (ESTA, eTA, ETIAS, K-ETA, UK ETA — not visas, but required before boarding) · E-visa · Visa on arrival, which may exist at airports and not at land borders · Embassy visa.
The Schengen rule
90 days in any rolling 180-day period across the whole area, which counts as one country. Leaving France for Spain resets nothing. Use an official calculator; the arithmetic is counter-intuitive and it is now automatically enforced.
Outside Schengen and useful for resetting: Ireland, the UK, the non-EU Balkans, Turkey, Georgia, Morocco.
Similar shared allowances exist elsewhere — the CA-4 agreement covers Guatemala, El Salvador, Honduras and Nicaragua as one.
Land borders
A different process from an airport, with its own frictions: opening hours, lunch closures, unofficial fees, fixers, and buses that do or do not wait. Cross early in the day, carry the fee in cash in the right currency, and check the stamp before you walk away.
The border guide covers it in full, and the visa guide covers the paperwork.
Political constraints on routing
Some sequences are refused: Kosovo from a third country before Serbia, evidence of Nagorno-Karabakh before Azerbaijan, Abkhazia from Russia before Georgia, Northern Cyprus before the Republic. Check the order, not just the countries.