As several have said, this will most likely be a case of complacency with the verification of the bottle during and after the gas switch. All of us are guilty of complacency with some aspect of just about every dive. It's human nature to go from caution to comfort to being too comfortable with the inherent dangers of the sport.
For example, during the student phase of training, the students are often so task-loaded with their first deco dives or so caught up in the steps of the procedure, that you can switch to an oxygen bottle at 70 feet and not one student in a 3 person team will correct the mistake. They don't see it because their minds are focused on how to verify that they fail to notice what they should be verifying. Once divers become familiar and skilled with a gas switching procedure, they are usually very good about verifying switches. After a while, divers may start to short-cut the procedure because some procedures may be so elaborate that steps are stripped in favor of speed and simplicity. But, the divers will still be sure to lifeguard one another and verify the switch. This may last for 150 or even 1500 dives, but eventually, most divers will become complacent.
Faith in one's buddy or team to make the correct choice during the self-verification steps might lead to a lack of attention over time. Experienced divers who are comfortable with a procedure may find themselves on auto-pilot during a switch. You could be thinking about a fight with your significant other, what your mechanic told you about repairing your truck or SUV, a shopping list, or anything that distracts you from truly acting as a lifeguard. As divers, we are so used to responding to the "Okay" sign that a diver can be a million miles away mentally and flash an "Okay" in response to the question, "Is this bottle correct?" Survival experts claim that being where you are mentally is one of the most important aspects of surviving any situation. Be where you are and pay attention.
If we are task-loaded during a switch, if we switch in poor or no visibility, if we didn't bother with a verification procedure, or if we remember being distracted during a switch, we should verify bottles again at the earliest opportunity.
I can picture a possible scene unfolding in this case. The team picks up the bottles expecting them to be correct for the depth. Being used to doing it, each team member self verifies that he is pulling the regulator from the correct tank, the one they just picked up, maybe without really looking at the MOD marking? Perhaps they didn't bother doing a team lifeguard check because they had faith in each highly skilled diver to make the correct switch and they were just lackadaisical at the moment or busy with other issues? Or, if they did go through the verification procedure, maybe one or more team members was on auto-pilot and watched the switch, but really didn't see and process what was going on?
Given the quality of this group, I'm sure we'll have a good idea of what happened in the coming days or weeks. But, given that we can speculate about what may have happened, despite procedures to prevent such a tragedy, in this early discussion I wanted to add:
When verifying a gas switch, don't just watch the procedure, but really see what's going on. Your buddy's life may depend upon it.
I'm not saying that this is what happened nor blaming any of the team - especially since we don't know exactly what happened, yet. In these forums we try to learn lessons from accidents. I just thought this might be a good place to talk about the fact that divers can watch a switch without seeing it. It's important to safeguard against complacency when you are lifeguarding a teammate. If at any time you have doubts about the gas or bottle being breathed - check and verify.
