Ziimbabwe - Zambia (and back)
Zimbabwe - Mozambique (illegally, the closest bar was just over the border, just waded over a river)
Zimbabwe - Mozambique (again, illegally, there is a lovely mountain range that spans the border, and there are no border guards hiking around)
Zimbabwe - South Africa (and back)
South Africa - Botswana (and back)
South Africa - Namibia (and back)
England - Scotland (not sure if this really counts 
)
Another zero-national-border-walker here.
I am impressed by all the Dopers who’ve crossed so many.
I, on the other hand, haven’t even walked across a state line. The highest is only a county line. (E.g., I had relatives whose backyard corner was on the county line. We kids went across that ton of times.)