When we first looked at our new/old house we liked it and it didn't take a second viewing to sign on the dotted line. However, when we accompanied the home inspector on his tour, we noticed a foul odor in the house. We made the incorrect assumption the methane gas smell was coming from a dried up p-trap. A little water to the suspected trap and a infusion of fresh air and the odor was gone.
When we next visited the house the odor was not present, so we assumed the best, and then, as we were moving in, the odor became overpowering. It turned out there was a broken sewer drain pipe, and I will not bore you with the details of the sleuthing it took to find the culprit. But eventually, after a couple of plumbers, the home inspector, the gas company emergency crew, the city sewer experts (?) and a host of other workers offered their opinions, we ripped into a basement wall and found this drain pipe.
For scale let me tell you that it's a 4" cast iron pipe and it had a half inch crack running the length of the piece. The crack was on the top half of the pipe, fortunately, because no sludge or waste material leaked out, only gas. Sewer gas contains a host of bad stuff that can make you sick or give you headaches at the very least. And it stinks. The thermostat would turn on the furnace and begin to combust air, and when that happened, because it's a tight house, the combustion would begin to suck air from the sewer pipe and spread it throughout the house. Dee-lightful!
All things considered, it could have been a lot worse. We haven't gotten the bill for the work yet, and it won't be cheap, but whatever it is we will gladly pay it.