Mildred tells Montag that she forgot to tell him about the death of Clarisse. They don't talk to each other like regular people, she is always busy watching the TV walls and has the Seashell Radios in her ears.
"Why Didn't you tell me sooner? Forgot. Four days ago. I forgot about it." (Bradbury)
In this society, people are not friendly in the same way that we are friendly with people, they socialize around the television, they don't get real personal, they are indifferent to each other. And, remember that Clarisse McClellan and her family were quite radical in their behavior, they would be considered dangerous by people like Mildred. She would want nothing to do with them, so it is easy to understand why she forgot to tell her husband.
"Clarisse is shown in contrast to Montag's wife, who totally accepts the values of the society, even when it is harmful to her health. Clarisse does not like the social activities that most people in the society like. She describes her family to Montag as liking to sit around and talk. Clarisse's family disappears later in the novel, and she is said to have been killed by a car."
Mildred fails to tell Montag about Clarisse's disappearance. There's a question as to whether she left with her family or was killed. Mildred's telling is fragmented:
No. the same girl. McClellan. McClellan. Run over by a car. four days ago. I'm not sure. but I think she's dead. the family moved out anyway. I don't know. But I think she's dead.
Her choppy sentences and repetition that she doesn't know show a few points about her character. One, she is clearly distracted at this point. Although she is talking to Montag, she also has a Seashell in her ear, and so is also listening to radio broadcasts. Also, her ability to remember anything that truly happens (as opposed to those things which happen on her TV screens) has been essentially destroyed by her addiction to "the family."
Montag, of course, is deeply disturbed by this information, and his future actions are somewhat dependent upon this exchange. He becomes reckless, and questions Beatty about the McClellan's disappearance, and ultimately turns his fire hose on the other firefighters in an attempt to escape his living hell.