Nothing really astonishing, but a kid we hired at the company I work for. The owners loved him, one of the owners wife knew him personally and vouched for him, the other employees at the location he worked loved him, thought he was super nice and a hard worker. He worked for us for maybe a year and made it all the way up to assistant manager of one of our locations. I told everyone, including the owners, that he sketched me out and that it was a matter of time before he fucked up, but everybody said, "nah, he's great, he's super nice, always show up on time whenever we need him, " etc. His "niceness" always felt incredibly fake and silver-tongued to me, but nobody else believed it.

One day he showed up to work, talking to himself the entire day, and eventually yelled at a customer, punched the wall, and got into a physical altercation with them. Turns out he'd been doing meth nearly the entire time he worked there and had stolen thousands of dollars from us. I never felt better or more vindicated than being involved in his firing and showing him the security footage of his fight with the customer. It's probably my all time greatest "I told you so." Bosses ask me to sit in on everyone they hire now.