Washing machine and appliance overflows
The rubber supply hoses behind a washing machine are under full house pressure every hour of every day, whether the machine is running or not. They are the most common single point of failure in a laundry room.
Hoses fail while nobody is home
A burst hose releases at line pressure until someone closes the valve. The standard advice — replace rubber hoses with braided stainless every five years, and close the valves when leaving for more than a few days — is unglamorous and it costs less than almost anything else a homeowner can do.
The standpipe backup
A washing machine pumps out faster than a partially blocked standpipe can drain. Water comes back up and over, usually onto a basement floor, usually during a wash cycle nobody is watching. Lint, detergent residue and, in older houses, a partly obstructed lateral are the usual causes.
The slow ones are the expensive ones
A dishwasher seeping under a cabinet base, a refrigerator ice-maker line weeping behind the unit, a laundry connection dripping into a wall cavity — none of these announce themselves. They are found by smell, by a soft floor, or by a stain appearing on a ceiling below.