You are in the vintage scuba forum, so I'm assuming you are talking about the old-time pendulum rebreathers using pure oxygen. Hans Hass used these for many years, but they used them in shallow water. Cousteau experimented with them, but almost died of oxygen poisoning twice, gave them up and used the experience to figure out a air open circuit unit with Emile Gagnan that he named the "Aqualung." The first air unit by
Northill was a pendulum unit, and it was rejected by the U.S. Navy because of the potential for CO2 buildup and difficulty clearing the unit of water; they subsequently invented the
Northill Air-Lung (LaRocchelle, Ed, "The Northill Air-Lung Regulator, A Collector's Review,"
The Journal of Diving History, Third Quarter, 2014, Volume 22, Number 80, pages 23-26). So yes, there is a potential with pendulum rebreathers for hypercapnia (too much CO2), as well as hypoxia (if the unit is not purged of nitrogen) and oxygen poisoning (if one dives an pure oxygen rebreather too deep--greater than about 25 feet).
SeaRat