Well I'm gonna sound really stupid because I've made the same mistake 4 times, you'd think I'd learn. On 4 separate occasions I've started to descend and as soon as a gulp of water enters my mouth I realized I never put my regulator in my mouth and was descending using my snorkel. The first two times I did it as a very new diver and thought for sure I was going to drown and frantically kicked up to the surface. The last two times I did it I calmly thought "you idiot", found my reg, spit out my snorkel and put the reg in mouth without even slowing my descent. So I figured I was still very stupid but becoming much more calm when I made mistakes. At least I haven't done it again in my last 100+ dives, maybe I've finally learned better.
Same here twice while teaching
I was a New Instructor, and was so preoccupied, I just descended, I don't use a snorkel anymore, It is now a fold up in my pocket!