Contradiction with Genesis 17:17
This verse describes Abraham laughing in disbelief at the news of having a child at his old age, while Genesis 21:7 marvels at the fulfillment of that promise.
Genesis 17:17: Then Abraham fell upon his face, and laughed, and said in his heart, Shall [a child] be born unto him that is an hundred years old? and shall Sarah, that is ninety years old, bear?
Contradiction with Genesis 18:11-12
Sarah laughs at the idea of bearing a child in her old age, doubting the possibility, contrasting with her later astonishment at actually having Isaac as described in Genesis 21:7.
Genesis 18:11-12: Now Abraham and Sarah [were] old [and] well stricken in age; [and] it ceased to be with Sarah after the manner of women.