I've had this happen twice. The first time it happened, an ISR fixed the issue. It happened to me again last week. It was a very simple PCB and I was 100% sure that everything was routed properly, but my Status report was telling me that there were a few unrouted connections, two of which did not match the net list. Any way, it was very easy to visually confirm the routing, so I generated the gerbers and had boards fabricated without a problem. If it happens again, though, I'll have to get some help fixing it.