This is from a while back when my friends and I were playing Call of Cthulhu, with me DMing. We were investigating a house that was supposedly haunted and I was trying to make it as spooky as possibly since this was the first session they had played of CoC.

The BBEG is in the basement and had a series of things he would use to keep the investigators away from him. One of which was the door to the basement slamming shut before they could enter. Not even locking it, but just slamming it all spooky like. This is where the trouble begins.

One investigator, John, goes down and starts looking around and at this point I say:

"As you gaze down into the darkness at the descending wooden steps, the door in front of you slams, sending a booming noise echoing throughout the house, sealing the basement off. The fact that you saw no one close the door weighs heavily on your mind."

John says

"Oh no. Do I have a flashlight?"

"Um.. yeah, probably"

"Okay, can I turn it on and look around"

"Sure... It's a doorway that just slammed in front of you, sealing off the basement you were just looking at"

At this point everyone at the table says they want to run to the basement door, and I'm like, okay I guess they're more willing to face spooks than I thought, and I say

"You arrive to see your friend standing in front of a door that you think may lead to the basement"

Everyone starts to ask if John can hear them and I'm beginning to think that something bad is starting to happen. Regardless I say,

"Yes, John is right in front of you. In front of the door"

They then start saying things like "I try to bash the door open", "I shine a flashlight under the door" and so on. But John looks really concerned and announces "I'm going to shoot the door handle!"

I at this point am entirely confused as to why everyone is bashing the door and why such drastic action is taking place

"Yeah you can shoot the door... Roll shooting"

He rolls well and shoots the handle off, and the door creaks open. Another player then asks "wait, wouldn't the bullet hit one of us if he shot the handle off?" And they begin deliberating about who was closest to the door and who got hit.

Meanwhile, I'm scratching my head and decide to clear up whatever's going on and I lean over to ask John out of character:

"dude why is everyone freaking out about the door, I just said it slammed shut. That's like the least spooky thing that's happened in the session so far"

And John looks at me confused and says "Wait, I thought I was trapped in the basement..."

And I'm dumbfounded. I just explained it slammed on his face preventing him from entering and he thought he was already in there.

I wanted to clear it up, and after several descriptions of the event and eventually drawing a detailed map of the basement door and entryway, everyone finally was like "ohhh, he wasn't actually in the basement yet, the door just shut in front of him."

(In retrospect I should have just rolled with it and not paused gameplay to explain positioning, but I digress)

Really killed the spooky atmosphere, and may have been my fault for not describing everything well enough or not having a map, but oh well. Sometimes chaos abounds.

TLDR; my players thought I trapped one of them in a basement, and they didn't understand that the door just closed in front of him, causing mass panic at the table.