Are there reports of ticket agents denying you for their lack of knowledge?

I'm flying from Bangkok --> Shanghai --> USA, and in all aspects I qualify for the transit-free travel. I'm mostly worried that the ticket agent in Bangkok will be unfamiliar with visa-free transit travel, since I'm guessing they rarely encounter people traveling using this, and it doesn't apply to Thai people. Friends of mine have had issues before with ticket agents denying them a flight to China, and it turned out the agent was in the wrong. Have you heard of this happening to people trying to use visa-free travel? Is there some official website that states this policy, in case I need to show them? Thank you!