I did a quick search, but didn't see anything specific to this issue.

I was trying to book a DCA-STL flight this morning for late May. I've been watching these fares for a few weeks and today was the best fare I've seen in a while. I pick my flights, and even make it to confirming my credit card info. However, when it comes to confirming the flight, DL.com pops up a message saying the fare has now changed and it went up about $40. I tried searching again and ended up finding the same low fares (on multiple flights) only to see it go up when I actually tried to confirm the purchase.

I called the medallion line and they in fact confirmed the higher fare. They have no idea why DL.com is showing a different fare. The CSR just said, yup that's the website.

$40 isn't a huge deal, but it's kinda annoying.