I accidentally made friends with someone I suspect was borderline last year. Met while studying, traded phone numbers since we were in some of the same courses and both struggling with one. After that, she is suddenly my best friend in all my classes, and we end up working on a group project together with another student.
She decided she didn't like the other group member and actively tried to exclude him from the project. Then the text messages started. The first couple were harmless and friendly, but they suddenly became extremely hostile when I didn't respond immediately. In the course of an hour I received two extremely rude and off the wall texts stating that if I wanted to be friends I had better change my attitude, followed by a dozen in rapid succession that just said 'hi'. The alarm bells went off, and I confronted her and said I was no longer interested in being friends and under no circumstances was she to send any more text messages. She apologized profusely and made her excuses, but did not change her behaviour. After more bizarre texts, outbursts of hostility and crying in class, and continued manipulative attempts to exclude the other group member, I ended up having to have the professor remove her from the group. She apparently had no clue there was a problem.
She never texted or talked to me again, and she dropped the class. I couldn't help but think that I dodged a bullet.