I run a multidimensional (Sliders-like) campaign, and every time the players do a transit they are given a momentary glance at another reality. My personal favorite of these has been:
For a moment you're all sat around a table. The smell of pasta and garlic hang strongly in the air. You have this sensation that, while you're here, you're family. Someone one sets down a basket of bread sticks... and you arrive at your destination.
My wife once played a Wizard with the Hermit background, and decided that the "secret she knew" was that she was a character in a game. She would periodically make references to older rule sets. Yes, THAC0 was name-dropped.
chandra-nyalaar says: "my favorite d&d thing is when someone flubs like a really obvious perception roll or something and the DM gets to be like 'well, you're pretty sure you're in a room but you could be wrong.'"
animar-smol-of-elephants says: "it is either really wet of really dry, you're not entirely sure"
ugin-the-spirit-dragon says: "One time a guy in our party rolled a nat 20 on a perception check, but there was nothing around he didn't already see, so the DM said 'You're not quite sure, but for a few seconds it seems like you're standing on a giant's table, surrounded by 5 giants.Your party seems to look stiff and fake,and large papers and dice are strewn around you. Then, everything goes back to normal.'"