I hate it when the "and this is where we end the session for tonight" part happens 10 minutes in... once had a high level party teleport to the complete opposite side of the map 10 minutes after starting... That is when I learned what high level really meant.
I had a 15 minute session once because I offhandedly gave my players a foldable boat like 15 sessions before. I had intended for them to have to cross a mountain range controlled by Dwarfs who'd ask them to assist with a dragon hunt in exchange for passage.
I had my players once fall into a giant underground lake that was home to a Dragon Turtle. The intent was that they would have to fend off the best while trying to get ashore because they were too low level to actually defeat it.
One of my players just so happened to have a Feather Token for a Swan Boat, which they used to trivialize the encounter and get to shore before the monster could reach them.
The only reason she had the Token was because I had given them all a few thousand gold to buy magic items with at character creation, and she had decided to buy a bunch of random consumables instead of the normal +1 weapons and armour. And the best part was that she was a very new player and probably didn't buy the Token with any actual consideration.