The hypothetical situation you describe is exactly what happened on a previous forum I was on (which was even smaller than this and relied on a smilar modding policy).
A quote was posted from a product review article (as in, 'hey, look at this' - not even masqeurading as something else), sans source, and the company that owned the copyright took direct legal action. It was a non-profit forum, with all costs absorbed by the owner - membership was open to anyone and there were no adverts, absolutely zero revenue stream. Despite asking if removal would settle the matter the copyright owner went ahead with legal action. The site owner had to settle with his own savings, which was very difficult.
I know this is the extreme end of the scale but some people are aggressive with any copyright violation unfortunately so I do a sharp intake of breath when I see things posted without appropriate reference as this one incident basically ruined the community. Unsurprisingly the owner decided it wasn't worth the bother running it after that episode.
