Many years ago a lawyer friend told me that if you don't get it in writing, you have nothing. If someone refuses to sign an agreement, you can bet they don't intend to keep it.
Verbal contracts may be "legally binding" but they are way more difficult to enforce.
We all understand (I hope) the importance of getting the deal down on paper and signed by all parties. I learned that early on. But it took a few thumps for me to understand that any and all documents related to the project need to be signed and dated as well. And that includes drawings.
I soon developed the protocol of printing multiple copies of working drawings and having the customer sign and date them all. A copy was left with them. Any changes also have to be signed in the same manner. It's the only way to protect yourself as well as the customer.