90 day visa on arrival are not automatic. Even if your citizenship falls on the list of countries that typically gets a 90 day visa, I would highly recommend checking the actual stamp placed in your passport at the port of entry. I was coming in and out of South Africa on my US passport and got used to getting the 90 day visa, but once I got a 30 day one instead. Had I not checked, I would have had problems if I planed to stay longer than 30 days!
Here is the list of countries that typically get 90 days on arrival:
Andorra, Argentina, Australia, Austria, Belgium, Botswana, Brazil, Canada, Chile, Czech Republic, Denmark, Ecuador, Finland, France, Germany, Greece, Iceland, Ireland, Israel, Italy, Jamaica, Japan, Liechtenstein, Luxemburg, Malta, Monaco, Namibia (90 days per annum), Netherlands, Norway, Panama, Paraguay, Portugal, San Marino, Singapore, Spain, Sweden, Switzerland, Tanzania (90 days per annum), Trinidad & Tobago, United Kingdom of Great Britain and Northern Ireland, British Islands Bailiwick of Guernsey and Jersey, Isle of Man, British Oversees Territories namely: Anguilla, Bermuda, British Antarctic Territory, British Indian Ocean Territory, British Virgin Islands, Cayman Islands, Falkland Islands, Gibraltar, Montserrat, St Helena and Dependencies (Ascension Island, Gough Island and Tristan da Cunha), Pitcairn, Henderson, Ducie and Oeno Islands, the Sovereign Base Areas of Cyprus South Georgia and South Sandwich Islands and the Turks and Caicos Island, United States of America, Uruguay, Venezuela, Zambia (90 days per annum), Zimbabwe