I learned my lesson about that the hard way back in the early 00s

I logged into LinkedIn, and it seemed like it was loading, but then the login failed and it prompted me to log in again. I tried and it didn't work, then looked more carefully.. and the second login prompt was actually a "Give us your e-mail login so we can scrape all your contacts and spam them" prompt.

Hoo boy, glad I was using separate passwords for everything