DNs with 2+ years of experience, what was your biggest mistake and how could someone else prevent it?

I’ll go first.

My mistake: I arrived overseas planning to use ATMs to withdraw cash for living expenses. I’ve been in this country before (cash society) and was able to make ATM withdrawals before, so I didn’t think anything about it. But between the first time and the second time, I switched banks…only to find out that my new bank couldn’t service ATMs in the country. I arrived with literally no way to pay for anything.