One where I was dumb was when I sold my then one of best friends a laptop and took his word for him going to pay a week later even though I knew he was a small time grifter. We were tight so I thought he wouldn't screw me over but alas, believing in people is dumb.
The second was an online friend of many years and good emotional support one way and the other, but then out of the blue the dude starts spewing hate and straight up tells me he's a nazi. Fuck that guy. Well fuck the first guy too, but at least I knew the devil back then.
As it's also fondly called, the "Fuck you, I got mine" phenomenon.