I repeat this lecture ever so often.
Whenever possible "Get It in Writing" - not only to have proof of agreements but even more to make sure that both parties have the same understanding of them.
And sometimes it also provedes the means to make sure that you and the person you're talking with have the same understanding of implicit assumptions.
The most compelling story to back this was told to by a tour guide in New York when we drove along 42nd street.
When William Van Alen was hired to construct New York's Chrysler Building he was so sure of himself - the tour guide told us - and so well versed in the trades of the craft that he didn't bother to get a written contract. After all 5% of the buildings overall building cost was a well established fee in the market
After the building was completed - beating the Bank of Manhattan in the race for tallest building of the world - Chrysler refused to pay Van Alen with words like: "We don't have a contract. You should take more care to always get it in writing". Of course I don't know if this is a true story or not, but at least it serves well to remind me of my own advice.
OTOH it's hard to capture everything beforehand and some thing are so engrained into our perception of the world that I also tend to put more trust in the co-operation than I should.
(BTW: The story itself is of course backed - at least somehow, although with slightly different numbers - by wikipedia in the last paragraph about William Van Alen's Life)