George Clooney had actually met Michelle Pfeiffer years before,as in the early 80s he had gone on a few dates with Michelle's sister actress Dedee Pfeiffer.
When Maggie Taylor forgets the kitten's name, it was not a part of the script. Mae Whitman actually forgot the kitten's name and stayed in character. The director thought that it was so cute that he kept it in the movie.
Near the end when Melanie bumps into the table in her apartment, it was apparently not in the script. Michelle Pfeiffer's clumsiness was therefore left in the movie.
On the way to the soccer game, Jack is holding Melanie and stomping in the puddles. His pants are wet and discolored to above his knees. Yet when they arrive at the soccer game (assume within minutes) and Maggie hands him her back pack to hold, his pants are dry and there is no discoloration.
When Michelle Pfeiffer is in the taxi with George Clooney's phone, he calls her up to give her messages. She then acts totally surprised that they mixed up their cell phones and she has his, but then continues to say he has a meeting at 4, indicating she already knew she had his phone by mistake because she took a phone call for him.