One issue I frequently see is clients not making a paper trail. The client will call me about a problem with their town, or someone they're dealing with on a contract.
The client will describe to me how they've talked to so-and-so on the phone, and the person says "x" and then doesn't do "x", or does "y" instead. For example, a town official tells them it's okay to build something, and then after they build it they come back and tell them they didn't have the right permit. Another example is when the other guy is supposed to pay you, but before payment he says he needs "x" documented. So you provide "x", and now he tells you he needs "y". Repeat ad nauseum.
Get it in writing! Or put it in writing. If you have a conversation about something important, and you have the slightest concern about the other person's reliability, send them a confirming letter.
"Thanks for speaking with me the other day. Following on your approval of this project, I will go ahead and start building. I really appreciate your assistance with my plans."
Notice the politeness. If you write in an aggressive tone, the person is more likely to respond in an unwelcome manner. This vaguely goes along with a line from a college buddy of mine -- "Sincerity is very important. If you can fake it, you've got it made."
Another tip is to include in your letter a self-addressed stamped envelope. Mention this in your letter: "Please let me know if you have any questions or concerns. I've enclosed an SASE for your convenience."
For extreme security, send the letter in a confirmable manner (FedEx, certified mail, etc.). Now they can't deny they got the letter.
If your problem goes to Court, your efforts to document the communication can be helpful. It will not always make the difference, and in some cases may not be admissible, but it will help in some cases. It's a lot better than: "But judge, he said ...".