What's the fastest you've gone from "I'll take it" to "absolutely not"?

I used to think apartment hunting was mostly about finding a unit with a good layout, decent rent and a solid location but after going down a rabbit hole researching a few places I was considering through Reddit, reviews, StreetSmart and whatever else I could find, I realized how much information about a building never makes it into the listing. It completely changed how I look at rentals now.
What's the biggest thing you've found out about a place before signing that made you immediately walk away?