D&D 5E You know you’re playing D&D when/if…..

[rgoodbb]

...one of the players correctly guesses the ending/puzzle/secret bad guy right from the start.

…the DM has to find a way to destroy a PC's 10 foot pole just to get the game actually moving

 

[Andvari]

...instead of asking what somebody's job is, you ask what class they are.

 

[AnotherGuy]

The DM asks for the marching order

 

[Laurefindel]

… you go to college to « improve your non-weapon proficiencies »

Or*

… you commonly refer to your real-life botches and fumbles as « rolling a 1 »

* if you get the « non-weapon proficiency » part, you played in an edition where rolling « 1 » on a skill check is a critical success…

 

[Krachek]

Players discuss way to long the numerous subtile ways to enter the room, to finally smash the door and rush inside.

 

[Andvari]

You investigate every inch of the floor for traps in areas where there's no logical reason for traps to exist.

 

[Stormonu]

…When returning to the tavern, you hum “Shave and a haircut” to oust the mimics.

 

[Andvari]

You don't investigate every inch of the floor for traps in areas where there's no logical reason for traps to exist, so you fall into a trap!

 

[Umbran]

...one of the players correctly guesses the ending/puzzle/secret bad guy right from the start.

Has nothing to do with D&D.
Here's an example.

I was running a game of Classic Deadlands - a "Weird West" game, with magic and steampunk superscience. The PCs had been on the right side of a conflict, but were asked to get out Dodge (literally, they were in Dodge City, KS) for a while on principle that maybe they should have let the Sheriff handle the ruckus.

So, the party goes a wandering. They wandered into a wide spot in the road that had a saloon, a bar, a general store, and a bank that had just gotten robbed. The safe door (the place wasn't big enough for a vault) had been literally ripped of its hinges, and the horse tracks from the escape just stopped dead in the grass about a mile out of town. Townfolk, being a superstitious lot, thought ghosts robbed the bank.

The group's mad scientist character pipes up, "Ghosts don't exist! But maybe it was like, a carnival sideshow act - the strongman ripped the door off the safe, and they had a horse tamer that had trained the horses to be picked up off the open prairie by a specially modified dirigible airship!"

Of course, the only thing he was wrong about was ghosts. They did exist, but didn't rob the bank. But the rest he was totally right on.

 

[GMMichael]

...when the fighter chopped an orc in half, the thief avoided detection to start unlocking the treasure chest, the other 5 orcs hit the fighter like a whack-a-mole, the cleric successfully resurrected the fighter... and you haven't done a single thing yet.

...when the DM carefully describes the shadow behind the pillars, moving and growing in size, filling the room with an ominous presence, and casting a translucent ripple through the air to overcome an ally's senses, and someone asks "do I see this?"

Of course, the DM's response is "roll Perception."