I went today to Mae Sai to renew my 'visa on arrival/exemption': Very smooth, very fast, no questions asked. The whole process (border crossing and return) took less than 15 minutes. The immigration officer even gave me a smile :-). Thanks you 'thai visa advice' for information. (to save money in the future, bring $10 for Myanmar visa, otherwise it is 500 Baht). Bus ride with greenbus smooth also: left CM at 8am, back to Chiang Mai at 8pm.