Updated CRM 2013 to SP1 UR3.
Following that subgrid column headers are misaligned vertically and a scrollbar appears on hte right. Just the headers, not the grid content!.
If I stretch the column, the issue is fixed.
I found another similar issue on the forum but it was caused by a CSS modification.
That post suggested that issue is on the label.ms-crm-List-Sortable class. There is an overflow:hidden property that is causing it to bring the sort arrow down and add the scrollbars at the far right.
