Myanmar did operate a true visa on arrival system for about 6 months where anyone could turn up and get buy a visa on arrival. this was stopped just before the elections in 2010.

As far as I know there is still a visa on arrival for the Siem Repp - Yangon flight by MIA. At least I have not heard anywhere that they have cancelled it.

For a long time they have operated and still do operate a pick your pre-arranged visa up at the airport.