I'm reminded of the time I GMed a scenario and didn't notice that there were 10 foot squares. I thought the building was just half it's actual size, because I thought they were the standard 5 foot squares.
I think it was pointed out to me in the GM prep thread here before I actually ran the adventure. At that point, I realized that the GM I'd played it with previously made the exact same mistake.
Since we're talking about how to actually play on the 10 foot square grid, I'll point out that when I GMed this, I used the map as drawn (10 foot squares), but just told everyone to treat them as 4 squares of 5 feet each. It meant overlapping minis squeezed into squares, and constant questions about "Which corner of that square are you in?", but it played just like any other 5 foot grid otherwise.