Although until now I have not come across Google-Plus-Four method (now I have copied that)
Very little these days is original and it must be easy to copy something unknowingly. Everything that you I or anyone else writes on here or anywhere else has, to some extent or other found the information form some sort of text or educational site or book. Subjects that are quite specific and can only be explained with certain words would be more at risk.
plagiarism is maybe a grey area, if a photographer sees a photo on a website or in a magazine of a location and then goes and photographs it for a publication, competition, or a coursework assignment although he/she would automatically be the copyright owner, it is to some degree a copy?
I don’t see a problem with the odd fraise but whole paragraphs or threads is copying the work of others.
Personally I would credit the info source or add a link to a thread if applicable.
I think it is a fine line.
This could be one of those never answered questions.