It is a fairly common mistake, in my experience, to forget to sort data on the split variables when SPLIT FILE is in use, or to similarly forget to sort data on the group variables for RANK, and so on for other forms of grouping.  At least, when I'm trying to use PSPP for tasks, I sometimes forget.

It would be nice if we could warn about this, so that users don't get too surprised, but it's somewhat difficult because we don't know the intended sort order.  The actual sort order could be any permutation of the grouping variables, and each column could be in ascending or descending order.  For N grouping variables there are 2**N * N! sort orders, so a brute-force algorithm is going to be too slow.  Also, we have to do it "online": we only get one pass through the data.

I have, however, figured out a decent algorithm that is both efficient and usually figures out when data is not sorted.  At some point, I should implement this in PSPP.  See http://groups.google.com/group/comp.programming/msg/c7ebefe24af2f930  for details.