Audio problem: When John Anderton's wife throws John's eyeball (stored in a clear plastic bag) onto Tim Blake Nelson's pipe organ, an ongoing organ note is heard, implying that the weight of the eyeball is depressing a key or two. We then see a shot of the eyeball resting on the organ's keys, none of which are depressed. (02:01:20)
Audio problem: When the child asks her mother for a balloon from the balloon man, the mom's words don't match her lips. (01:36:50)
Answer: Because subverting the expectations of the viewer makes it more interesting. The audience (and potentially John) are set-up to expect bad things, which don't happen. Once the "bad thing" happened, the suspense would be gone and everyone could relax. Expecting something bad but knowing when it might happen maintains the tension.