To Beat a Dead Horse
Geoffrey Pullum, returning to a subject both he and I have discussed before:I would have thought we could agree, whatever our feeling about re-using phrases we've enjoyed before, that it only becomes plagiarism when an unattributed passage of non-trivial length is used with the dishonest intent that the borrowed passage should be incorrectly thought to be original. The conjunction of those boldfaced elements should be regarded as definitional, I think. (See my earlier ruminations on plagiarism here and here and here.)