The most common mistake? Assuming that all embassies/consulate deal with the same information, in the same way, rather than knowing that different embassies can have different rules.
Couple that with not double checking the information which is auto populated when submitting passport into the system.
Whether or not both will need 500k, or if one will need 1m baht, will depend on where you’re applying from also, due to different embassies having different rules on that.
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