If you just list the index number then the keys are not unique:
<template>
<!-- keys will be 0, 1, 2... -->
<div v-for="(row, rowIndex) in rows" :key="rowIndex">
<!-- keys will be 0, 1, 2... -->
<div v-for="(cell, cellIndex) in row.cells" :key="cellIndex">
</div>
</div>