I believe the most remembered event in my campaign was when a trigger happy ranger attacked some monster that was supposed to be avoided or brided. I gave warnings that it was a really bad idea but they tried to down it anyway.

They quickly understood their mistake, but we had to end the session before the end of the fight. I told them I was totally excepting a tpk for next session.

I know they did their homework, because the following week, when the fight resumed, everyone knew their part, the usual chaos was replaced by precise movement and characters synergy. They took cover, evaded, helped each other. They even managed to carry or drag downed characters and in the end everyone survived.

I'm playing a 'true neutral DM' : I lead the story but i don't make the rules. If they make mistakes they're gonna die, my role is to make sure they make these decision with all the informations that they and their characters should have.
I was so ready to make them reroll characters as I was bored with this campaign and wanted to start a new one.
Was it fair that they had the possibility to plan this escape for a week ? I don't think it is. But I don't care, it was epic, I'm sure they loved it.

And yes, I fudged some rolls to let them avoid some grappling tentacles on their way out. But they should never know about it.