I had a near panic a few weeks back.
The Situation:
Now diving in Cold Water, so dealing with Drysuit and and steel tanks (plus as I'm working under regulation and have to carry a Pony: no, I'm not happy about it). we dropped slightly down stream of the shot and I proceeded to motor towards it, not too strong of a current but enough to cause me to exert myself. I reach the line last and head down; at this point I'm breathing heavier than normal so descent is slow because I'm not relaxed, however I prefer to find my mojo underwater rather than on the surface (personal choice). I reach the bottom of the line; 9m, and shocking viz: the team just visible only metres away, then we begin our search (archaeological diving). All the while I'm focusing on regaining my breath control, yet I can feel the lizard part of my brain wanting to 'freak out', which is bizarre as after 2000+ dives I'm fairly comfortable in the water.
The dive was fairly unpleasant the whole time; 30 mins. I was on the end of a swim line, so basically on my own save a small length of cave-line telling me there was someone at the other end, and fighting to keep a CO2 hit a bay from the early shallow breathing. We surfaced without incident and went home. But it was a dive I'd rather forget.
In a bizarre postscript:
On this dive I had been using one of the team's personal steel 12s, also on a dive prior to this I had come up with a pounding headache and nausea using his tanks. I assumed 'bad air' but really couldn't smell anything when I checked them post dive. I had done a breathe check pre-dive as well, but couldn't taste anything. The same was true of this dive with the near panic. When we took both cylinders to get filled, the compressor operator picked up a bad odour straight away when double checking the valves were clear of moisture. He gave them to me and another to confirm this. It was odd that in the comfort of the dive shop; away from the sea and the smell of the car or boat, the smell was overpowering!!
What I took from this incident is how easily a panic attack could have overtaken me had this happened when I was new diver. The sensations were awful and it was only the experienced part of me; telling me to calm down, that kept the incident at bay.