Carbon monoxide is an odorless, colorless, tasteless gas that is a byproduct of incomplete combustion of anything that burns.

You could be standing in a room filled with carbon monoxide and never know it.  If the carbon monoxide level is extremely high, you could fall unconscious in minutes, and be dead within an hour.

CNN produced a short, but good video on carbon monoxide poisoning.

Heat exchanger cracks

… by themselves do NOT cause carbon monoxide poisoning, however they are dangerous. This video illustrates the effects of large heat exchanger cracks on the burner flame. This situation is dangerous for a number of reasons. First, there is a potential for fire. Second, the wiring inside the furnace will be burned/melted. Third, some amounts of carbon monoxide will be released into the area around the furnace. Note that the CO is coming out the FRONT of the furnace and is NOT being introduced into the indoor air stream of the blower.