I'll put my two examples:
1.) I used to work with a really lovely lady on reception in a dentist. She had come from another dental practice and told me that her colleague there also went for the job and she hadn't got it so it was really awkward for them to work together after. I asked our employer about this, just in a 'by the way why didn't you hire so and so's colleague' sort of way. He said that they were going to but someone put a letter through their door with loads of negative comments about her so they went with my colleague instead! I knew that it had to be my colleague who had written this as we had all gone to the employers home for the interview and that was where the letter was posted, not the business address. Never trusted her after that, not properly.
2.) I made friends with a nice mum at a baby massage class. We met up a couple of times but I soon started getting FB messages from a random lady in Portugal who told me that nice mum had stolen her husband and left her pregnant and alone in a foreign country. I did try to be normal after but it seemed a bit strained as I so wanted to ask her about the messages but was too shy. I feel a bad now about not trying harder to stay in touch but these messages made me feel uneasy (that the nice mum might have a stalker more than anything!) if it was now I would ask her and tell the woman to do one (even if it was true.)