This is the bit people should be worried about, people seem to think Dual Tax treaties exist so an individual can CHOOSE which rules to follow.
Take a person living on an income just below the income tax threshold in Europe (perhaps a state pension) but this person is not in Europe! This person for tax purposes is resident in Thailand and his income is above the tax threshold in Thailand perhaps making him liable to pay tax in Thailand.
Having paid no tax in Europe, he does not have anything to credit against his Thai Tax liability.
Not intended to be a qualified response, but thought provocing all the same