I had a relationship with a guy in my 20s who was obsessed with the idea I was a cheat and hated me seeing my friends. One day he told me he had received an anonymous letter telling him all about how terrible I was, flirting all the time with different men at work and so on. It was a three page letter torn out of a spiral bound notebook. He told me it had been sent to him at work.
I knew it was a load of old pony because nobody I worked with was sufficiently interested to do that, nobody knew where he worked and it was all untrue anyway. I asked him to show me the envelope it had arrived in.
He came up with a Jiffy bag and it was his own handwriting on it in capital letters. He had written it himself using his non-dominant hand and sent it to himself. Absolutely ridiculous.
Of course this is mildly amusing not to mention stupid, but the guy went on, a year later, to beat me up and tell me he was doing it to restrain me as I was in danger of hurting myself.