In treeviews, column headers recently (2.10?) started appearing in the header for each column. This is rather confusing, because the arrows should indicate the sort order.

However, it is possible that a column will not be sorted. For example, if you sort your music by track title in Rhythmbox, the Artist column will not be in any order. In such a case, the arrow is confusing and doesn't tell anything about the column.

Off the top of my head, it might be useful to indicate an unsorted column some other way (though I know not how).