Warning Warning Warning
REBREATHERS CAN AND DO KILL. YOU'RE THE ONE RESPONSIBLE FOR YOUR OWN ACTIONS. YOU MAKE YOUR OWN DECISIONS AND ARE THEREFORE RESPONSIBLE FOR THE CONSEQUENCES.
You can die from either too much oxygen or too little oxygen. You can drown. Screw up and you are apt to die. When you enter the water, you are on your own, even if a buddy is nearby. You alone must make the decision that you are capable of using a rebreather and you are up to the dive as well as your equipment being up to the dive. Even extensive open circuit experience will not prepare you for the details contingent upon staying alive underwater on a rebreather. If you have any concerns about your abilities, either take a course or elect not to dive. If you are foolish enough to dive rebreathers, be aware that it is a case of practical evolution in action. It is an effective sorting out process that has created many unhappy widows, mothers, and children. Many rebreather users, including myself, recommend against it. The following pages of my rebreather project are described for your entertainment only. It is not my intent that you build one of these and go out and injure or kill yourself (going "Tango Uniform" in the parlance of my US Military friends here on Okinawa).
Also, Dave Sutton has a warning in much the same light as the above on his "Franken-Son" rebreather page (Franken-Son Has Arisen! A KISS awakened the Sleeping Bear), LEGAL ADVICE FOR THE STUPID.