When a person from a western country wants to enter Thailand, he will discover that it's actually several sets of rules he must relate to. As each border-crossing have their unique visa rules.

At the "Kingdom of Suvarnabhumi Airport" one can arrive without visa, a stamp in the passport will be sufficient. But you can only stay 90 days out of the last 180 days doing it this way.

At the "Kingdom of Thailand-Cambodia border" you can not enter at all. If no visa in your passport, you need an onward ticket leaving within the next 30 days. I read this ticket should be a flight-ticket when I was at the border myself. And the officer also claimed a flight ticket. But some say a bus ticket will do. Probably this varies depending on where you are on the Cambodian border.

At the "Kingdom of Thailand-Malaysia border" no one seems to know and no one ever asked what the rules are. You just have to try your best.

What I would like to know, is this common practice around the world?

To have such an ambiguous rules for entering a country?

(surely there must be some experts out there)

At the "Kingdom of foreigners who have non-immigrant visas or re-entry permits", everything is fine and dandy as usual.

...What I would like to know, is this common practice around the world?

To have such an ambiguous rules for entering a country?

The rules are not ambiguous, but not every person entering Thailand is checked every time if he complies with all the rules.

I do not know about the whole world, but I know many countries where this is common practice.

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Maestro

After answering the same question for you four times over a two month period we must consider this to be a troll; and as such is closed.