Recently I ran Mines of Madness as a 5e one-shot for some teens.
Scott Kurtz and Chris Perkins serve up some silly-hard fun with a comic and deadly tone much closer to White Plume Mountain than Princes of the Apocalypse.
So Lander, the party's curious thief, buys it spectacularly in the very first encounter--massive damage, no death checks, no Spare the Dying ... none of that. Just ... dead.
Given how hard it is to die in 5e, the players were surprised to say the least, but after a moment of gaping they got right back into character.
"Should we say some last words or have a moment of silence or something?"
"I guess so."
"I hardly knew him. We weren't very close, but then a purple worm ate him instead of me and I feel like we've had a lot more in common since then."
"That was beautiful."
A bit later, a significantly more cautious party encountered a cockatrice.
"What do we do?"
"Well, I could try to shoot it from here..."
"No! I can't remember if you turn to stone if you look at it, or if it looks at you--and that really matters right now!"
They decided not to risk it and headed off to explore in a different direction. Reed the Halfling, a nominally Lawful Good fighter / tank electing to linger near the back and leave risks to others despite various bonds and flaws that painted him as fearless and doggedly loyal ("He's going through a phase; it's just something all halflings go through.").
Encountering a hall full of undead dwarves, the now very cautious party decided to use a distraction to try and slip past them rather than wading into combat. Reed's player scanned her character sheet:
"This is for all those haters who said you wouldn't need an iron pot!"
The distraction bought them the time they needed and the explorers went on to avoid several other encounters before meeting their doom.
It wasn't until the end of the session that we realized we'd played for hours without a single combat since Lander got swallowed back at the very start of things--that initial pulse of extreme danger had caused the players to shift gears and focus on exploration and survival rather than just bashing monsters.
Have you had similar experiences where that first encounter set the tone?
Do you find that all your halflings go through that stage too?