My booking.com latest experience: booked big appartment for 4 people. Arrived to destination (Bristol, UK), and apartment already had guests inside. Tried contacting landlord, no reply. Called booking com, they offered acommodation 30km from the city centre, and its already 11pm, no way to get there. Had to pay our own hotels, and we never got money paid to booking.
One neighbor of that apartment said they often double book! Seems booking com doesn't care.