Example? I ask because you may be assuming that the "original" source you found on the Internet was not written by the same person who "plagiarized" their own thoughts later.
No ... I don't want to give examples. For several reasons ... like one person said ... they may not realize that it is necessarily rude or wrong. I think it is, but I concede it is probably, most definitely a pet peeve of mine. Also, I don't want to put anybody on the spot and possibly emabarass someone or cause hard feelings. If you want examples, I am sure you can find them.
I do understand that some people on this site have their own webpages or have their writings published elsewhere. That is not what I am referring to.