That's the main problem. The person at the gate has absolute power so you will get stories of travellers getting away with shonky paperwork for border after border while another with immaculate paperwork will strike a heap of trouble.
Places like Iran pose special problems such as guide requirement for certain nationalities and carnets.
Whether to carry an International Driving Permit is something else you have to decide.
Guess you know about rule changes for visiting the US after visiting certain middle east countries.
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