I lucked in to seeing a fare booked with points drop by 600 points. (The account had a zero points balance and may never benefit from the drop, but why not make the change?) Completed the change, checked points detail and saw the account had been charged only 300 points fewer than the original cost. Immediately called SWA and was told I apparently caught a midnight PDT fare change in progress. That seemed a bit odd, as it happened about 00:40 PDT. Fortunately, the SWA agent was able to adjust the points price to the amount quoted. ^

Looking at the fare quote page afterwards, I noticed that attempting to book the $114 ($99++) cash fare resulted in an "OOPS we are unable to secure the price..." message, but attempting to book the equivalent points fare resulted in no such message.

Customer-friendly bugs are zapped pretty quickly, so this won't last long, but if you find you are a bit late booking a great sale fare, try booking it in points and calling for a price adjustment when you are charged too many points.