Insurance company not paying claim

I thought I was covered through the end of July '09, but I get a letter saying they paid a prescription claim July 10 and want me to pay them back for it since I wasn't covered. I have a certificate of coverage stating that I was covered for July, but the insurance company said that was sent by mistake. I called the administrator (it's COBRA) and they also say it ended July 1. Going through financial records to track down premium payments would be next to impossible because there was so much confusion with adding and removing wife and kids and getting the ARRA subsidy, I don't even know what I was supposed to be paying, that's why I lost the insurance. Anyway, everybody's records show I was insured until July 1, but my memory and more importantly my certificate of coverage say I was covered until July 30. What else should I look into? I've asked for an account summary from the administrator, but with all the confusion I don't know how much good that will do. Is the certificate enough to be right in refusing to repay the claim?