OK, this was me.
I did a dive with a single & pony. then, during the break, on shore, I chatted to fellow divers whilst chainging my cylinder. I decided that, as the plan was for a nice shallow drift, I would leave the pony behind. Loaded my kit onto the boat and went.
As I kitted up, I had forgotten to turn my air on, and got someone to do it for me. checked the breathe, but couldn't figure out why it was only registering 80 bar.
Jumped in anyway, and thought well, it will be a shallow one, so i still have a good 1/2 hour.
What I had also forgotten to do was add a bit of weight to compensate for the lack of pony.
20 mins into the dive I was at 50 bar and too light to stay down. Luckily, by this point, I was only at around 6m and didn't come up too quick.
What I should have done was think Oh, stuff. forgot to change the cylinder, so no dive.
Actually, i figured out later that I had simply put the old cylinder back on because I wasn't concentrating and giving the dive the proper respect.