Q:

How does clogging tailpipes result in carbon monoxide in the cabin?

Reports like this of people dying from CO poisoning in idling cars due to "clogged tailpipes" are disturbingly frequent.  But how can this happen if the car is outdoors?
First of all, how does a modern engine respond if tailpipes are blocked?  If they are obstructed enough that exhaust can't escape the system why doesn't the engine stall?
Second, since the engine does in fact somehow continue to run, how does exhaust end up in the cabin?  I'm guessing that there's an obvious weak point at which the backpressure escapes the exhaust system, and that point must be near the cabin air intake?  (But if that's the case, would setting the cabin airflow to recirculate prevent the intake of exhaust?)

A:

One way I could explain it from that photo, is that there will not have been any snow under the car, so the exhaust fumes will have easily been able to exit from the tail pipe and spread under the car.  The fumes will then have gone into the engine compartment and from there could have been sucked in by the fan.
Alternatively the exhaust could have had a hole anywhere along its length which once again would have allowed fumes underneath the car.
If in perfect condition, there should be no where for fumes to exit the exhaust, other than from the tail pipe.  If the tail pipe was totally blocked, the engine would slow down and stop.  However, there is no way snow would be able to block the tail pipe sufficiently.
Putting the heater on recirculate may have limited the intake of fumes, but a car's passenger compartment is not air tight, so the fumes will still find a way in if the car is totally surrounded by fumes as it will have been in that photo.

