I have previously made mistakes quoting posts, but it is usually when trying to trim down 4 or 5 embedded quotes, not when quoting a single post as in this case. Additionally, it is in general obviously meant to be a quote because I am then writing something in direct reply to the quote.

I agree with everything you've said, particularly about not punishing honest mistakes, but there are two things that are working against him here:

1 - There is no unclosed quote tag, or even bits of a tag, so after hitting "quote" he would have to have precisely deleted the before and after tags

2 - He added nothing of this own to the post, which you generally do after quoting someone


Edit:

Actually, looking at his post again, he's quoted someone else correctly first, before the alleged plagiarism underneath. Therefore, he can't possibly have misquoted the second post. It must have been a copy-paste job.