It is common practice to refill the spot when people cancel. If they had a client call for an emergency situation the may have been told the schedule was full so they couldnt be seen. Or sometimes they have a call list in case of a cancellation.

I am t's last person of the day. My time isn't the greatest but it is doable. When the person before me cancels she contacts me to come in earlier.

Yes it does suck when things like this happen....