As often happens, we each have a piece of the story, and both pingouin and Sylvar are basically correct. Linotype operators who made a typing error would often run their fingers down the two middle columns of the keyboard to cast "etaoin shrdlu" on the next line. The resulting slug was intended as a 'flag' to call the compositor's (the fellow who assembled the pages for printing) attention to the erroneous line so it (and the "etaoin shrdlu") could be discarded (actually recycled, all Linotype type was melted down and re-used), but sometimes under the pressure of a deadline both the error and the "flag" would be missed and printed in the paper.