Yes they do. It quite literally happened to me, which is why I wrote that comment.

I had to wait at the gate in the hopes there would be a no show. I wasn't the only one waiting, either. There were two of us. "Good news", they told us after everyone else had boarded. "We have seats for both of you."

They hand-wrote me a ticket and I walked onto the plane, only to find that the seat they'd assigned me was already taken. I stood in the aisle for about 10 minutes, clutching my bags, the only person still standing, until a flight attendant came and told me there wasn't actually a seat for me and that I needed to leave the plane.

When this has happened before, people assumed it was overbooking but it was actually a plane change.

I would have loved if they had an actual excuse for me, such as a plane change. All they could tell me was that I was late to check in, so they couldn't guarantee a seat for me on the flight.