DM: "you smell the distinct smell of gas building in the cave."
Player: "I cast fireball."
DM: "Alriiiiight... As the sparks ignite in your hand, the gas explodes, causing a cave in and burying the entire party under a mountain of rock. Congratulations, you are dead."
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.
My favorite saying is that "you find no traps" β "there are no traps." Or more in line with this meme, "there's probably a trap here somewhere but I'm not gonna tell you what the DC actually is."
"There is nothing there." -> "You don't see/perceive anything."
Even if there isn't something there, keep the players on the edge and guessing how good their skill worked. And it makes it just a little bit less gamey if the character doesn't have some divine insight that there really is nothing there instead of deciding that their skills just show that there is nothing there and be happy with that - or not.
Of course, extend that to everything. Pull away from the facts of the game and put more ambiguous language in.
players proceed to analyze the door for an hour, not realizing if they take the stairs slightly to the left, they'll be taken to the treasure room they're looking for
I've gotten impatient before and snapped "the door rots away from the time you've spent arguing and falls open" which just sparked a few minutes of debating whether the building is sentient and will eat them if they enter the door.