As I scroll down with two fingers I sometimes see a row of white or blue pixels between two correct rows of text pixels—rows that come from the same text line. The bad pixel rows appear upon scrolling the text down. In the blue pixel case (as in the referenced .pdf file) the pixels that should be black are blue instead and other pixels of the row are white. Sometimes I see a row of white pixels between two rows of text pixels. I think in both documents the .pdf file is just one column wide. The attached .tiff file was made from page 3 of the document. It shows 4 rows of blue and white pixels that should be black and white. I scrolled when the Grab utility had focus. Magnification = 125%. Clicking "Report bugs to Apple" causes the bad pixel rows to be fixed. WebKit Version: 6531.9, r48199
This was a common pattern on my old Mac. I have not seen it after a few months with my 2.4GHz Core 2 Duo.