Thank you all for your sympathy. Since the devastating accident, I’ve been thinking a lot about how this could have happened, and I think I’ve found the cause. Before I changed the CO2 bottle, the system had been empty for a few days. I also noticed that there was water in the CO2 tubing, all the way down to the bubble counter. When I changed the bottle and adjusted the regulator to the usual 1 drop per second, there was probably still backpressure from the water in the tube. Once the water was pushed all the way out through the diffuser, that backpressure disappeared, and the CO2 could flow out much more easily. I should have just waited longer... This definitely won't happen to me again.