It sure is a natural reaction to panic when things go wrong underwater, but you just can't or you may die. I remember when I was getting my basic, they did buddy breathing. I was sharing a regulator with this girl the was also getting checked out and when I handed her regulator back to her, I couldn't find mine. I then had to buddy breath for real and we both went into a panic as we surfaced.
Last time I went diving, I had cheap rental gear and found it very difficult to breath through my primary, so I thought I would switch to my reserve. The problem is that the reserve did not work and I had to switch back to my primary. In the process, my BCD(which gets modified by the language filter if you don't use the D) somehow filled up with air and I ascended about 50 feet before I was able to get my buoyancy under control. I was able to continue my dive, but had to make an additional safety stop as a result.