This happened to me today, as a new JJ diver with probably 50 hours in the unit. It was pretty much completely my fault due to skipped check lists and poor equipment config.
Yesterday I did two dives on the unit (about 50ish metres each) and was having issues both times. Struggling to get down, high dil usage. On the second dive, my buddy noted that my OPV on my dil first stage was leaking.
Because I'm an idiot, I 'solved the problem' by replacing the misbehaving OPV with a port plug. Just until I could get a replacement OPV, I told myself.
On the boat at 5am this morning the unit was misbehaving. It kept blowing the wing LPI off. I replaced the inflator on the boat and jumped in.
At 60ish metres 25 minutes into the dive the ADV free-flowed and very rapidly filled my CLs. I thought it was the misbehaving inflator and quickly removed the LPI. Then thought it might be a stuck solenoid but the PO2 was constant (the solenoid was firing to maintain PO2 as the dill rushed in, making the problem worse).
It took me a while to find the dil valve and shutdown. I was sufficiently flustered by that stage that I bailed out and we ended the dive uneventfully. It was a scary experience, and I felt fortunate I was with good dive buddies. Hitting the flow stop (which I initially forgot, going straight for the valve) wouldn't have helped, for reasons we diagnosed on the surface.
It turns out that the seat on the dil first stage was damaged, and the IP was high, and climbed once the valve was opened. This is why the LPI was popping off, and this is why I'd been burning through Dil the day before.
If I had not removed the OPV, this would have been a leaky dil reg, not a 'boom'. Dive likely would have ended, but under far better conditions.
If I had followed the checklist, which requires using an IP gauge to check the IP pressure before each day's diving. The problem would have been fixed before I got in the water.
In the end, we had about 45mins of deco and my first real life bailout went ok. I'll be re-drilling my shutdowns (including more aggressive venting or even releasing the loop to let the gas escape in an emergency situation) in future so that I'm better prepared in the unfortunate event this happens again.