Originally Posted by RobOnLI

It usually means that one or more of your segments is operated by a different carrier and that the flight you wanted from that second carrier wouldn't confirm.

I had this happen to me once before because of a DL flight thrown into a CO reservation. I purchased online and then got an email saying that my itinerary would be confirmed soon - but that there might be a credit card issue. Rather than waiting for a follow up email, I called the Support Desk and they told me that the DL flight from ATL-SAV was showing overbooked and they couldn't sell me a ticket on it, even though their website says it's available. What they wound up doing was giving me a SEA-IAH-SAV flight (which should have cost $350 more online) for the same price I purchased the ticket for if I had flown the DL segment.

-RM