...when you honestly believe that for some reasons or another, your brain decided to have a vacation with out you knowing about it. Kinda like those days when for some reason, it seems like you had lost your smarts from the previous day and became somewhat of a dumb ass. The days when you decide to eat everything within sight, and then go to the grocery store, and get some more. The days when after weeks and weeks of your friends trying to get you to do something that you would normally never do, you decide "Hey, what the hell. I'll try it!" and something horrible happens. The days that you'll remember for the rest of your life, and kick yourself in the ass for.
Those days are what make people, people. The reason being is that when we make mistakes or do something out of the ordinary, we gain something in return for whatever may have happened. That thing we get is knowledge. Knowledge to know never, EVER do that thing again. It's like when we were kids and the stove was on. Rather than tell us not to touch it, our parents just stayed silent as we slowly reached for the extremely hot metal/stove thing. Then we touch it, and it hurts like hell. You ask your parents why did they let you do something like that, and you know what they said? "Now you know not to touch something that is freaking hot."
The mistakes we make in life help build us up. It shows which paths not to take, and which ones are much better to travel. So even if you know it is a bad idea, you should at least try it once just so that you know that it was a really, REALLY stupid idea in the first place.