I got it on my first Death Ride. I had the classic symptoms, most notably bloated stomach. And riding really slow.

But that was an extreme event- 10 hours in very dry air, and for at least half of that the temperature was around 100 degrees. I was drinking the organizer-provided sports drink ("Revenge") and water, but no additional electrolytes.

Since then I have determined that on long hot rides I need some electrolytes. I'm a "salty sweater" and will finish those rides with salt on my skin and in my clothes. I know other people who don't sweat out so much salt and don't need electrolytes for the same rides.