The Quran Verse
Surah 11:71:
And his wife was standing, and she laughed. Then We gave her good tidings of Isaac and after Isaac, Jacob.
The Relevant Source Text (The Bible)
Genesis 18:12:
So Sarah laughed to herself, saying, 'After I am worn out, and my lord is old, shall I have pleasure?'
In Genesis, Sarah laughs after hearing the promise of a child because she is skeptical of her old age. In the Quran, she laughs before the news is even given.
This is a minor but telling "narrative slip." The Quran keeps the "laughter" (which is the etymological root of the name Isaac), but detaches it from the logical context of the story. It shows that the author remembered the motif of the laughter but forgot the reason for it, resulting in a muddled retelling.