In Mediawiki version 1.18 there is something different then in 1.17 regarding sorting on two columns in a table. In 1.17 it was possible to first sort on column A and then on column B. Then it gave the results sorted on column B, but when there was a same value in column B in multiple rows it sorted on the information in column A. It doesn't do that anymore. On http://nl.wikipedia.org/wiki/Gebruiker:Basvb/Bug I builded a example-table.
If you sort that table on column 1 first an column B second. then take a look at the C's. In the first row from the C's there is 1,2,4,3 (instead of the logical (1,2,3,4).) On en-wiki it works fine and shows 1,2,3,4 (see http://en.wikipedia.org/wiki/User:Basvb/Bug)