I know there's been previous threads about this sort of thing, but I can't find it at the moment.
Anyway, I just received this envelope in the mail today with no postmark. Not especially unusual, as it could have just slipped through the canceller, but in this case it actually has the bar code (and the backstamp orange bars too), so the piece clearly went through the mailstream for delivery. In fact it is the first piece of stamped mail I received since the 45c postage rate went into effect and no postmark!

I had always thought that these marking were applied during the same pass through the mailing equipment, but apparently that is not the case if this sort of an example is the result.
I know in the scheme of things, one stamp doesn't mean all that much, but it still seems quite counter-productive that the USPS has just increased postage rates to 45c, yet doesn't bother to cancel stamps that are legitimately placed on a piece for mailing. In actuality, the USPS loses 90c (the 45c lost by delivering this piece of mail but not cancelling the stamp, and the 45c lost when the stamp is reused on another mailing) for every such error. Assuming this happens many times, it seems as though it can quickly add up to thousands of dollars or more in lost revenue on any given day.