Backlinks from non-similar sites

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Okay, I know a fair bit about SEO, read the books, put it into practice and get good results. But I saw a response on here about backlinks which made me wonder. The response basically said "Seeding links indiscriminantly will result in an adverse effect." This would make sense but we do not have full control over back links. Yes we can go to similar sites and request back links. But what happens if your site starts getting back links from unrelated sites which you have never visited. If the response is correct then it would be easy to cause a competitor serious problems by just creating back links to their sites on unrelated sites! So can anyone clarify whether getting indiscriminant links will have an adverse effect or will just be ignored?