Can't properly rebook passengers after kicking them off their original flight

United recently rerouted me because my original flight was overbooked. Travelling from Tulsa to Calgary through Denver, I ended up flying to Las Vegas from Denver, then to Calgary. However, they couldn't figure out their computers in Denver and didn't actually book the second leg of my new flight. When I arrived at the gate in Vegas, I was by Air Canada that I had no booking and I was stranded as their flight was full. After much running around Las Vegas airport (between two terminals!), United eventually fixed it but didn't even apologize. In fact the Las Vegas agent was really rude the entire time and acted like it was all my fault.

I didn't mind the delay due to overbooking; this is the nature of flying and it happens. I didn't even mind that they mixed up my re-booking -- this happens sometimes too. But what really got to me was the agent's bad attitude. A simple "I'm sorry we screwed up your whole day and stranded you for hours." would have made me much happier about this. On leaving the United desk at Las Vegas, I said "you know, I just want to point out that although you helped me today, you didn't even apologize for all this inconvenience". All Ms. Kubo said was "Next customer!" So rude.