The bag tag could have been generated from a customer at the counter before the OP's wife, and by mistake the agent might've grabbed it thinking it printed when checking her in. In my experience, the agents usually check IDs against the boarding passes printed, but as WineCountryUA pointed out, we all make mistakes. Sometimes agents have caught their error mistaggign my bag (before sending it on the conveyor) and a couple of times I've caught it.