player rolls perception

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."

Player: "Why didn't you stop me?!"

DM: "You failed your wisdom check, dumbass."

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.

Instead of "that won't work" say "you can certainly try"

Don't put this out there, I can't let my players see this!

i was looking at the "all" tab and didn't see the community name so i thought DM stood for "direct message" and was quite confused

I have to pee I have to pee I have to pee I have to pee

My favorite saying is that "you find no traps" ≠ "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."

Say: "Interesting, why don't you roll STAT and see?" Mean: "I have no idea, I'm improvising everything, why don't you roll and we'll see?"

Something I got from Spoony which I really liked:

"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.

"Are you okay? This is the fifth time you went to the bathroom this session."