I've run into this as well. I called Delta (was PM at the time) and told them what was going on, and that the fare was showing available then sold out, and that when I re-attempted the search, that same fare kept coming back up. The phone rep was able to replicate the issue. After the usual platitudes of "we can't guarantee a price till you click "purchase" etc. etc." when I informed them that the old price kept coming back up and they confirmed that the price I saw was the price in the system, they did a manual override and honored the price.

Short version: if this happens, call 'em, and they'll honor the quoted price if they can replicate the issue.