Originally Posted by SamuelS

From what is being described above by other posters, it sounds more like a fairly wide spread bug rather than inventory changing. Not too dissimilar to how the fabled award calender can show "low" but dang it if you can actually book it half the time

If inventory changes, that's one thing. But if DL is (through system issues or otherwise) advertising fares for which at that time there isn't actually any fare bucket inventory available to allow booking of that fare - I'd say DOT would frown on this.