The concept of plagiarism by mistake doesn't really make any sense to me. I don't believe that a person can really forget to mention the source from where he just literally copied and pasted something into his post/comment unless they don't care, or aren't aware of the fact that plagiarism is treated as a crime in here and is dealt with a harsh punishment.
When the "mistake" or "didn't know it's bad" argument is brought up I'm trying to imagine the user shoplifting and then trying to explain to the cops that it was a mistake or that they didn't know it's not cool to walk out of the store without paying.
So yeah, it doesn't happen accidentally. And it's not like in the store we see signs "don't go out without paying".