Back when I was younger I probably cock blocked myself more than anyone else. Hindsight and what you learn with age and experience is a hell of a thing. Thinking back and realising, "oh wait she totally was into me." Realising you cared too much what people thought, was in your own head too much, oblivious to things and all sorts of stuff like that.
As for a cock block, one that still annoys me is I had a bunch of fat women ruin a relationship for me. I knew them, they'd talk about the one hot girl they work with and her terrible boyfriend. They convinced her to break up with him, which was the correct thing for her. They then had vicarious fantasies about if they were her and all the slutty behaviour they do. All the men they could sleep with and so forth. Yet that wasn't her, she was sweet, but a bit naive and not the brightest person.
I met her at something, she literally took my breath away. I kind of ignored her because she was so pretty. I had nerves. We eventually got chatting and we hit it off amazingly. To us no one else was there, the rest of the night was just us chatting and we kissed. We then started dating and the fat bitches went insane. I was too nice a guy, she isn't ready for a relationship, she is using me and she'll break my heart. It was really condescending towards me. I knew the situation, we got along great, we had great sexual chemistry and were just going to see where it went. Yet her "using me" became the focus of the workplace.
Everyday at work those bitches were gossiping at how terrible she was. Then they'd be in her ear. I was constantly having to talk her down as they would work her up. I then had some travel where we couldn't communicate, back in the days when wifi wasn't available everywhere and the fat bitches convinced her to end it and she should be single. Who knows where it'd have gone, but we should have been allowed to figure that out and those bitches should have let her be.