No, it generally doesn't. There are two levels of error correction built into the DVD format [data or video, doesn't matter, it's all bits]. PI Failures (PIFs) indicate the failure of the inner error-correcting code; this is where the second level takes on. PO failures are where the second, outer error-correcting code fails.
However, even if an error scan has PO Failures, it doesn't mean that the disc is unreadable. The ultimate test is to try to copy the data off the disc and let the drive do its work. When the drive hits the point of a PO Failure that caused CD-DVD Speed to give your disc a zero quality score, it will try reading the damaged area at progressively slower speeds, eventually slowing down to 1x speed, and retrying multiple times at the slowest speed. You can also tell the copying software to retry after the error dialog shows up... sometimes that helps too.
Bottom line, if the reading process completes and doesn't come up with a fatal error in the middle, you've got a perfect digital copy of the data. Reburn it [on better media, hopefully] and you will have the same 4.7+ billion bytes, or 37+ billion bits that you started with.