The pressure in the average pipe in a residential building is about 14 gallons per minute. That is a lot of water, and you can just imagine what would happen if that water leaked out inside your home. While it is not a very common occurrence, there are some situations in which it can happen. One of the reasons why it happens is someone cutting into a wall without checking properly what is behind it. While copper pipes are fairly solid, a power tool can drill into or cut into a pipe fairly quickly if you are not paying attention.
In a situation where something unexpected like that happens, it is easy to do the wrong thing. Trying to block the water or collecting it is not going to work, since the pressure is so high. Your best bet is to find where the shutoff valve is and turn the water off.