I dispatched with a 1 pack operation restricted to FL250 one day and on descent with the flight attendant asking what we wanted to drink was awarded with a take off warning horn. I looked at the copilot because he had been pulling tricks all day wondering how he did it. He looked confused so I thought, WAIT! that sound is for loss of pressurization too but I had never heard it. Looked up and sure enough, the cabin was above 10,000 and climbing. Putting on the bad pack fixed it so we didn't have to add power to the working side. Some cooling equipment outflow valve had stuck open letting the air leak out. No big deal but it sure gets your attention. I am happy we didn't get the problem enroute and continue to a higher altitude as is allowed.