Unfortunately, the old oil tank is still in the basement and has a small leak.

A friend of mine had some issues with leaks from a bad install of some stone siding.

Finally it was determined that there is a leak in the water main coming from the street. 

Leaks in roofs or windows that have resulted in mold (often enclosed in a wall and not visible). 

There were a couple of areas where the roof leaked and this caused water intrusion issues and mold.

The existing drain that the gutter down spout should connect to runs into the basement but is filled with concrete (not me) from the outside for some reason(it was prob leaking somewhere). 

Write into a lease tenant handles repairs, and you get back a rental with no repairs done, pails placed under sinks for leaks.

The home inspection will assist you during your due diligence period (assuming you negotiated this in the contract) in finding red flags... ex: roof leaks, evidence of past flood, etc...

You can't inspect any leaks or electric without it on 

When he got there he found that the bathroom on the second floor had been leaking and water accumulated in the first floor ceiling and finally broke through the dry wall spilling water onto the wood floors below.