Avoiding minor plagiarism is an occupational hazard of writing. There are only so many ways to say "Trichet told a press conference that monetary policy would continue to be tight for the rest of the year"; if you weren't at the press conference, you're going to end up using some close variant on a phrase that probably appeared in half the copy filed about it. To whom do you attribute it, if anyone?
This, however, is not minor, and also, not hard to avoid. I've very much enjoyed some of
Joseph James Twitchell's work, and now it's clear why; he stole large chunks of it from some of my favorite writers, like Virginia Postrel and Grant McCracken.
I'm really not very sympathetic to writers who do this, and then claim that their note-taking was somehow at fault. I can recognize my own writing from 100 yards away, even if I don't remember having written it--when I go through my old blog, I don't need to look at the author line to discern which was written by Mindles, which by me. Indeed, writing style is so consistent that I can finish long-forgotten passages in my head before I've read to the end. I find it very, very hard to believe that Steven Ambrose, Doris Kearns Goodwin, and now James Twitchell read these passages in their notes and then thought that they had written them. It's plausible to me that a particularly vivid phrase might occur to you as if you had coined it. It is not plausible to me that you accidentally remembered several paragraphs of someone else's work.
This sort of thing is particularly sad because Twitchell has written some vivid and interesting work--I particularly remember his descriptions of taking his wife and daughter shopping at luxury stores. It's not that he can't write; he chose not to.
Update Nick Gillespie's thoughts are, as always, well worth reading.
Update II Broader thoughts on plagiarism from Glenn Reynolds.
Update III James, not Joseph. Joseph Twichell was Mark Twain's close friend.