Leaking Supply Pipes 

Gradual oxidation and disintegration (Rust) of steel in pipe wall. Usually small rust spots will identify where pin hole leaks are beginning. They usually will seal up temporarily as the rust expands in the hole.

Remedy: Replace Entire pipe.

Electrolysis

Electrical current traveling through pipe causing the area with the positive charge to act as a cathode and the area with the negative charge to act as a sacrificial anode causing metallic ions to be pulled off the pipe by departing electrons.

Remedy: Make sure the ground and neutral in the main circuit breaker panel is properly connected to a copper plated 9 foot ground rod.

Burst Pipe

Plumbing in un-insulated/unconditioned space is subject to freezing.

Remedy: Replace burst pipe and wrap with foam pipe insulation.

Broken Pipe: Usually a section or end of pipe is not adequately supported and breaks off at threaded end.

Remedy: Replace section and add support.

Leaking Waste Pipes

Leaking P-Trap Under Sink Due To Disintegration Of Brass In Pipe Wall

Remedy: Replace with new P-Trap.

Leaking P-Trap Due To Impact From Objects Pushed Under Sink.

Remedy: Retighten P-Trap clamp rings and avoid hitting pipe in future.

Leaking Toilet Around Base And Floor: Usually caused by either loose or broken closet flange bolts or failure of wax ring seal.

Remedy: Unscrew or cut off  toilet bolts at base, remove toilet, replace wax ring, replace closet flange bolts and nuts, reinstall toilet.
    Note: pour one pint of bleach into bowl, wait ten minutes and then shut off water at wall valve and then flush.This will help reduce the virility and population of pathogenic bacteria in the waste line and your exposure. Be sure to wear rubber gloves. Wad up a large bolus of newspaper and use it to temporarly plug the hole in the floor while you are working on the toilet as the sewer gas can make you and others sick.
 

Leaking Toilet Between Tank/Bowl Connection
Sometimes the toilet tank becomes loose due to old disintegrating rubber washers oxidizing bolts or movement of the tank.

Remedy: Replace rubber gaskets, bolts and avoid bumping, moving tank.

Water Continually Dribbles Into Toilet Bowl From Tank

Usually the rubber flapper is old and cracked and is not sealing. Sometimes the float valve that fills the tank is not adjusted properly (water level should be appox. 1" below refill tube) and the water flows over the top into the bowl.

Remedy: Replace the rubber flapper.  Adjust the toilet tank float valve to shut off the water about 1² below the top of the refill tube.