CO2 poisoning is insidious, and can go unrecognised, due to its cognitive effects. It's a known risk in using rebreathers when scuba diving.
The UK HSE has published a video of a real-life incident where someone experiences CO2 toxicity from a rebreather (the victim survived and recovered) [1].
There is a difference between high and low concentrations of CO2 and reactions. In high concentrations you get similar effect to lack of oxygen (as it completely replaces oxygen in the air), in low concentrations it prevents the HbCO2 from unbinding (only 7% of CO2 in air is enough to do that IIRC) and causes silent death. There are stories of miners sitting down for a lunch and never standing up again because of that.
The UK HSE has published a video of a real-life incident where someone experiences CO2 toxicity from a rebreather (the victim survived and recovered) [1].
[1] http://www.hse.gov.uk/diving/video/co2video.htm