Todd Leonard has been sharing what little info is available on the Cave Diver's forum. Apparently, a deep bottle was dropped at the 70' stop, instead of the 70' bottle. At 120, the switch was made onto the 70' bottle, and it was apparently breathed for about an hour before the seizure occurred.
What is really sad is that there are very methodical procedures to prevent this from occurring -- dropping the wrong bottle, yes; switching to the wrong gas, no. There is as of yet no description from the dive team as to HOW it happened, but I have my suspicions. I know what I have seen happen, even on our "little" tech dives, and that is that, with familiarity, gas switches are treated with less respect. On our last tech dive together, I was upset with my husband for rushing the gas switch, and not stopping to watch each other as we should. His response was, "We only had ONE bottle." I pointed out that we had both 50% and 100% bottles on the boat, and there was no guarantee that we had grabbed the right ones -- the final insurance against ox-tox IS the monitored gas switch, which is why we are taught to do it. An extra 60 seconds at switch depth is cheap insurance.
Another thing is that the gas was breathed for an hour before the seizure. One of the things David Rhea beat me up about, over and over again in my Cave 2 class, was failure to include my buddy and his state of affairs in my situational awareness sweep. I was missing little things like markers hanging out of a pocket -- but here is a case where someone noticing the wrong MOD on the bottle might have saved a life. (Of course, if he was tail-end Charlie on the team, that wasn't going to happen.) It's HARD to remind yourself to look at your buddy from time to time with a critical eye, especially when you know them and have total confidence in their competence. But it's important.
If there is any lesson from this death, it is that mistakes can be made, even by very experienced people, and we have to remain vigilant against complacency or letting procedures slide, even in familiar dives.