This happened to me a few years ago once. I let it go at the time because I was much better than the guy and he was the type that might freak out about me not letting him have a first serve. It recently happened again, so I'm wondering if somebody can quote the rule on this situation. My opponent hit a first serve that was out but was close so I played a return. The ball hit the fence behind the server and he proceeded to hit his 2nd serve which was in and I returned it. The ball from the first serve went under his feet and he called a let giving himself a first serve.