My guess is that a cracked heat exchanger is pretty common and it's a pretty rare case where combustion gases end up in the house in any significant quantity. I could be very wrong, but if every heat exchanger in my furnace was cracked and I didn't know about it, furnace inspection didn't pick up on it, and CO detectors didn't know, than either I just won the lottery, or this happens frequently.
As an interesting tidbit on how good CO detectors are, I would like to tell you a story.....I put an addition on my house. That included adding to the basement. At some point, I needed to cut a hole from the basement in the addition into our existing house. I rented a gas powered cutting machine. The floor of the addition had already been installed, so I was working in a closed in area with a gas powered machine for an hour or two. I had box fan set up to get as much air in through the window as possible as I knew there was risk. With the wood framed wall and poly still in place (the basement was framed and insulated - this is Canada), I managed to set off the CO detector on the opposite side of the house even though I was working on the other side of an intact vapour barrier. The CO had to have traveled through the cut in the concrete, through the insulated wall, found it's way around the vapour barrier (probably in the floor studs), risen to a level sufficient to set off a CO detector that was 45 feet away. Yet that was insufficient CO to knock me down on the opposite side of the wall where the CO had to be 100's of times higher. I've had CO poisoning before (working on a car), so I was watching my vitals pretty closely as I was doing this job. I was/am truly shocked at how good CO detectors are!
