Our target was at 40 metres depth, since I was only diving on air we descended quickly. Everything went well until we reached 37.7m when my regulator suddenly started to freeflow. Since I was unused to changing to a second regulator and switching off the malfunctioning one, I signalled to my dive buddy that I had a problem. After a while he realized what was wrong and helped me with the switch. Unfortunately, this took some time and I lost a lot of air, so we decided to abort the dive and ascend back to the surface.
Even though it was a strange and scary experience I stayed calm throughout and kept breathing normally. Now I know that I have to practice switching regulators before going on another dive in cold water.