Something so unexpected or outlandish that you shut down for a second. Usually without clarification from the speaker.
Bill: "I was eating these really good chips that I found at Dan's house but I didn't know the name. turns out I was actually eating his cat's shit"
Hank: "..."
Bill: "...so I was at the arcade the other day..."
Hank: "What the fuck dude? you're just gonna drop a mental flashbang like that?"
Ariana: I think Mikey is high..
Justin: Nah, she is having a Vietnam Flashback from when she was on Hannah Montana.
Selena: I get it. Disney is awful.
Ariana: Yeah, Nickelodeon is better.