This one is fairly obvious so I guess this is not a dupe. Anyways...
On IE8, when the rows (or cells) of a grid don't all have the same height, the rows are not rendered correctly (inconsistent cell height and/or incorrect cell border position).
To reproduce, use the Cell Editing Grid example. Edit the "Common name" column of any row and insert one or several '<br>' tags in the cell.
Here's a screenshot demonstrating that: