My flight from LAX to MEM had S- before my last name. Normally the name on the boarding pass apprears like this:

LASTNAME/FIRSTNAME

but what I got was

S-LASTNAME/FIRSTINITIAL

and I didn't get the mileage credit for the flight but I got credit for a connecting flight after that flight that didn't have "S-" on the boarding pass. I got an upgrade by standing by on the flight that has S- and they changed my boarding pass a few seconds before I boarded.

Thanks for any comment.