Smoke detectors.
Since June of 2001: At least one smoke detector on each habitable floor in a home.
Smoke detectors required in each bedroom and in the immediate vicinity of the bedrooms (so, one in each bedroom and one outside of bedrooms. Immediate vicinity is typically defined by smoke detector manufacturers as within 13 feet of a bedroom. One smoke detector in a hall could provide coverage for multiple bedrooms).
These detectors must be interconnected, such that when one sounds, they all sound. They also must be powered by house AC current and have a battery backup.
When does this become a problem?
Home owners replace detectors with non-interconnected type or
not wired properly to allow interconnection. And, sometimes detectors are not set up properly when a new home is constructed.
Carbon monoxide detectors.
Since June of 2010: At least one CO detector is required if the home has
an attached garage or any fuel-fired appliance or fireplace. At least one CO detector is required to be installed with the immediate vicinity of bedrooms and inside of a bedroom that contains a fuel-fired appliance (such as gas logs).
CO detectors are typically combination type built into smoke detectors.
Older homes are not required to update smoke/CO detectors to meet modern requirements unless modifications to the home have been made that would warrant improvements (i.e., extensive renovations, etc).