Water pipe leaking
The other day I arrived at an inspection to find the water meter had been removed. Though this is nothing new lately, it is impossible to perform a full inspection without water. The selling Realtor thought the water was on (whoops)
So, I revisited a few days later after the new meter is installed. I turned it on at the street and there was plenty of water running inside. After 5 or so minutes I started loosing water pressure inside. I looked outside toward the water meter and saw water gurgling up through the ground.
Another good reason to make sure all utilities are on prior to the inspection.