I fasted for a day when fighting with my parents. Worst thing to do since I am a Type-1 diabetic and ended up with Diabetic Ketoacidosis the next day and a near-death experience :-(
Well it's pretty much for any mammal except type-1 diabetics --- plus folks taking medication for blood sugar / insulin regulation without informing themselves thoroughly beforehand and monitoring carefully.
I was an idiot and I am not proud of that! I was thinking like "I need insulin when I eat food. So since I am not eating, I don't need insulin and everything will be fine" :-(