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.
Mistakes are done by everyone, I agree, but why don't we commit this kind of mistakes? (I'm talking about everyone who are considered to be good members of the community). Of course we all require to search the internet in order to answer a few queries that we are not sure about, at least I do, but what I do is either confirm the thing first and then convey it in my own way, and if in a case where I need to quote something, I make sure I mention the source I've taken it from because I have some sense and I know it is not right.
The only excuse I can think of by someone who has plagiarized anytime in past (either by mistake or purposely) is that they were not aware of the rule. For that matter, The Registration Message thing should be rolled out which would be sent to every new account upon registration which would either contain a list of the rules or the link to the thread that contains them, along with the other important stuff a member needs to know before using the forum. Also, the message should be sent to every user account at the time of implementation to avoid getting complains and later excuses from the pre-existing user accounts that are still unaware of the rules.
I believe the current punishment of permanent ban for plagiarism has nothing wrong in it since it rarely happens when a user unintentionally plagiarizes and doesn't even realize their mistake even after reading so many things about the issue in the forum, and committing mistakes like that even after reading the rules sent to your inbox personally should be treated with the same punishment as others. Hence, I voted for, "No - Nothing should change and permabans remain".