What’s something someone did that instantly made you lose interest in them, even if everything seemed perfect before?

Being nasty to waitstaff, just because.

Attempts at social circle enmeshment despite warnings of that being not okay

Projectile vomited

Ridiculed other societies, other people. Or just having unusually strong opinions on trivial things that tend to reflect displaced emotions for something else. They tend to be signs of unresolved frustrations are indicators of future judgemental behavior towards you.

Disregarding past conversations of agreed upon behaviour. Disingenuous apologies as well

Ghost for a few days and then come back with a lame excuse

Drastic Personality changes from person to person. I know we’re different towards different people but I’ve seen some shocking changes where I felt like I didn’t really know who this person was. I didn’t know if they were two faced or what. Talking smack behind someone’s back is also a bummer.

Inconsistent. Their words and actions don't match and when I sensed that he doesn't even know what he wants

when they pulled a major red flag without even realizing it, like being super rude to waitstaff or going on a weird rant about something petty. instant turn-off.

Meth

Narcissism and/or one-sided friendship. I met this neighbor who is a perverse narcissist, and he tried to befriend me just to use me. I had to end the relationship pretty quick, as I could tell he didn’t see me as a person, but as a tool he could use. There are tell-tale signs for narcissism, and since then I’m much more careful about how people behave and talk. These people are very dangerous.