Altitude Bust?

I was recently on a United Airlines flight from KSEA to KDEN and discovered that channel 9 of the in flight audio has the cockpit communications between the crew and ATC. The in flight magazine says that the channel will be broadcast at the discretion of the captain. So, I dialed in and started listening prior to push back. After takeoff the flight was handed off to departure and the controller assigned the flight an altitude of 9000' feet. The pilot manning the radio acknowledged 9000'. After a few minutes I felt a sudden nose down motion and that rolling stomach feeling that you get when you suddenly stop climbing and apply heavy nose down pressure. I thought to myself, either there was a traffic conflict or the pilot just blew through his assigned altitude. Sure enough, one of the pilots called ATC and said something like "Approach, United xxx confirm altitude". The controller came back with "United xxx, descend and maintain 9000' ". I'm not going to pass judgement on the pilots seeing that I wasn't in the cockpit, but it's nice to know that even the pros make a mistake occasionally.