As the ATT U-verse installer lady was leaving, I immediately got on the phone (as described here) and cancelled my Comcast service.
At that time my bill was due, so I asked for a final amount. I wrote a check for that amount to Comcast and also wrote a letter explaining the situation, that I was canceling my service effective the 4th of March and I hoped they wouldn't screw up the bookkeeping AGAIN. I put it in the mail with the check.
Guess who I heard from over the weekend? Comcast sent me a bill for service through the next month and said I was past due. This exactly what I expected of Comcast. It's why I got the final billing amount from them and that's why I wrote the letter explaining that I was canceling the service that I included with the bill.
Pay heed to this kind of thing. Never give someone access to automatically draft funds from your account. You'll see offers for electronic bill payment that sound very convenient, but if you don't watch these guys like a hawk, they'll bill money from you after you've canceled.
This is how I ended my letter to Comcast when i sent my last bill:
ATT will no doubt disappoint in the future and whether I’ll consider returning to Comcast or not is wholly within your control. Specifically, I expect another billing error, whereupon I’ll get another bill for some ridiculous amount. I hope this low expectation will not be met.
Congratulations Comcast, you did not disappoint.
Update: Comcast replied below positing that this bogus bill was a reasonable mistake. However, this hypothesis is falsified by the two facts: The bill in question was dated after my conversation with Comcast and the bill showed receipt of my last payment.
The amount of that last payment was the figure that Comcast gave me over the phone. They could not have cashed my check without simultaneously receiving my letter notifying them in writing that I had canceled service.