Following W3C advice, it is recommended that tables have row headers. This is mandated in the English Wikipedia at Wikipedia:Manual of Style/Accessibility/Data tables tutorial.
<th scope="row">...</th>
Following rMW9e363d0e8a12f4fc205de08958622d6e8e250c81, table headers with row scope now have incorrect formatting. The sortable icon (the arrows pointing up and down) appears in each row header, although it is disabled and doesn't actually do anything. You can see this behaviour, for example, in List of teams and cyclists in the 2015 Vuelta a España on the English Wikipedia.
From T126110:
Unsortable headers in sortable tables now have a disabled arrow symbol next to them. This applies to header cell in the footer of a table as well as multiline table headers. You can see examples on https://de.wikipedia.org/wiki/Hilfe:Tabellen_f%C3%BCr_Fortgeschrittene#Zellen_von_der_Sortierung_ausschlie.C3.9Fen The disabled arrows in all those tables should not be shown.
Note that this makes some tables much wider than they should be, e.g. https://de.wikipedia.org/wiki/Fu%C3%9Fball-Bundesliga_2015/16#Tabellenverlauf (below are some more tables with disabled arrows where there shouldn't be any).