Floods in the laundry

We picked up a new (old i.e. 2nd hand) washing machine last night from his workmate. Much newer than our current one. So he installs it and does a test fill of it - looks as though its going well and comes in for a coffee. Much later in the evening he suggests I go out to see what's happening. What's happening is that there is a huge puddle of water on the floor, more like a small pond, not yet a lake. Turns out that if you don't connect the hot water tap, when the cold water goes in it comes out the hot water tap!! We live and learn.