Patoux01,
I'm not sure if there have been multiple accidents, or if I've heard the same account from different sources with different details, but there has been at least one death of a diver attributed to use of a pony tank. The two stories (or two versions of the same story, take your pick) I've heard are:
1) A diver lost his regulator during the course of a dive. Performing an arm sweep, he recovered what he thought was his primary regulator but what was in actual fact his pony regulator, which had not been properly secured. When the pony bottle was exhausted, an out of air emergency ensued.
2) A diver entered the water, thinking he was breathing his primary regulator but actually breathing his pony regulator. When the pony bottle was exhausted, he removed the regulator from his mouth. Thinking there was a problem with his primary air supply, he decided to switch to the pony, then correctly located and identified and tried to breath from the pony regulator.
These problems are made possible by four closely related equipment configuration choices:
a) The pony is mounted by attaching it to the primary cylinder, which can be done with any BC and minimum skill. However, it is difficult if not impossible to reach the cylinder valve during the dive.
b) Because of the difficulty reaching the valve, it is left open.
c) For simplicity, an SPG is not used on the pony.
d) Hoses for the pony and main gas supply attach to their respective 1st stages in the same area, contributing to the possibility of breathing the wrong reg.
Carrying the pony as though it were a slung stage is not possible with all BCs, and it's not as convenient, but it is much harder to mix up regs:
a) the pony reg is clipped off to the pony cylinder, making its purpose clear.
b) hoses are held in place by rubber bands or bungees and cannot be snagged during a lost reg sweep when endeavoring to recover the primary
c) because the valve is immediately accessible in this configuration, the cylinder may be carried valve off, which reduces any freeflow risk as well as reducing the risk of breathing the wrong reg. Even if carried valve on, the valve can be closed as a test if there is doubt as to which reg is being breathed.
d) An SPG can be used in this configuration if desired providing one more piece of confirming data as to what's going on.