I have recently learned a very important lesson. Always codify the changes that clients have requested. If a client requests something over the phone (or even over an email), always, always, always send them an email saying something to the effect of:
"As per our conversation, these are the changes that I will be making..."
And then LIST THEM OUT. This way, 3 months down the road, when a client complains that something wasn't done, you can simply point out that you did exactly what you said you were gonna do AND you did exactly what they SIGNED OFF on. And, you have the paper trail to back it up.
I guess the real lesson here is to always have a paper trail. Always have accountability explicitly defined on something that can be printed out and passed around.
Now, this might sound like this is just for covering your butt in business. Yes, that is a big part of it. But there is more to it. By forcing yourself to get in the habit of reiterating the client's needs back to the client, it forces you to think it out (thereby gaining a better understanding of the work to be done) and it gives the client another chance to say "No, actually you didn't understand... this is what I want."
It's really a win-win situation. Learn from me. Don't get in trouble.