From the Thai Embassy site: “Tourist Visa Exemption Scheme
To be eligible for the visa exemption scheme, visitors must meet the following criteria:
* Hold citizenship from an approved country
* Hold a passport with at least 6 months of validity remaining
* Pledge to enter Thailand strictly for tourism purposes
* Provide verifiable proof of accommodation in Thailand such as a hotel or an apartment address
* Provide proof of funds to support their stay (minimum 10,000 THB for single travelers and 20,000 THB per family)
* Provide proof of a return ticket for exiting Thailand (open tickets do not qualify, and the ticket’s destination cannot be a neighboring country of Thailand)
Visitors may be asked to show their travel ticket upon entering Thailand. If they do not possess a ticket proving their exit from Thailand within 60 days of arrival, they will most likely be refused entry.
There is no set entry limit for visitors entering Thailand by air, but visitors who enter Thailand by land or sea border and hold normal (non-diplomatic) passports will be granted visa-free travel to Thailand only twice per calendar year.”
So, you may be asked for proof of your ticket for leaving Thailand.
Another site said that doing a ‘border run’ then coming back, can’t be to a country adjacent to Thailand.
You also only get two visa exemptions a year when entering by land.