Technicalities are not an issue. It's a bit more murky when it gets to intent (was the user blatantly copy-pasting, or paying homage?), which is where mod discretion comes in.
It's quite murky. It's easy on random shit-posting baboons in pay-per-post campaigns. Say I had forgotten to include that source today. Would you have banned me? Note: Even though I wear a signature, I am not required to do anything thus making 1 or 1000 posts makes no difference. Financial motivation is a no-go here, and I most certainly did not intend to pass that as "my own work". So?
Hard to say without an example but probably not, if I can see any indication that you're not intentionally passing the text as your own words. I'm assuming you weren't just copy-pasting a paragraph of text and hitting "Post". Anything in quotes, quote tags, indented, bolded, following a colon, etc should be given the benefit of the doubt, even if the source is not explicitly stated. Then there is context and common sense. If you paste a Shakespeare piece it's not going to be plagiarism regardless of formatting or references. If you paste a paragraph from Coindesk though... better not do that.