It is unfortunate that so many take such a light attitude toward this. As you said, sometimes there are errors in the original work, and it would be nice if the author could be informed about it. Tracing back to the originator is often very difficult. And of course, there's the issue of simple courtesy, to say nothing of honesty.

I guess a lot of folks didn't quite understand the sarcastic intent of Tom Lehrer's lyrics, "Plagiarize; let no one else's work evade your eyes."