Two things are happening here.

Configuration » Product Listing » Display Product Listing Default Sort Order is null.

1) When I click on a product category the "sort" variable is not provided in the URL -- this is correct. However, when this happens, the Product Listing Descending Sort Order string is displayed after the "Item Name" column header, even though it is not sorted that way (it seems to be sorting by date added or ascending name, not sure which because I only have two items right now).

2) At this point the Item Name header anchor points to "sort=2a" but the anchor's alt text is "Sort products descendingly..." These do not match.

Once a sort order is selected from any column heading, these behaviors correct themselves.

Running 1.3.8a on UNIX (not sure what server, might be Apache but I don't think it matters).

Obviously when a default order is specified it follows that correctly, these only occur when that is null.