Here's a new-to-me error message:
# We are unable to price the flight you selected. Please select a different flight or call 1-800-I-FLY-SWA (1-800-435-9792) for assistance.
# The next lowest available fares for the flights you selected are listed below. If you accept the new price, please click "Accept and Continue" to proceed. If not, click "Modify Flight(s)" to make new fare or flight selections. Click here for additional information about our fare combinability rules which may apply.
The fare buckets shown at the front end aren't matching the availability from the back end. Weird. I think I'll try it again in the morning.
This reminds me all too much of Alaska Air's web site, whose front end display of fare availability was often more then 24 hours old. You could only get true availability by phoning them, which defeats the cost saving concept of having a web site.
Edited to add: It appears that I was trying to book a flight just as a $5 fare increase was being pushed through. The front end has caught up now. Bummer. Maybe I'll wait for a Ding sale. I can't remember the last time I saw fares change on a Wednesday night, and before midnight Central as well. Thursday and Monday nights are the usual change times, and near midnight Pacific time.