It meant that the linotype operator made a mistake and ran his fingers down the keys etaoin shrdlu to alert the printers to cut out that line. The linotype operator corrected the mistake in the line below. If etaoin shrdlu got into the paper, somebody missed cutting out the line. I used to see it in the Seattle P-I quite a bit.

SHRDLU was created in 1968.

T is the most common letter in the english language

Letter frequency in the English language - e t a o i n s r h l d c u m f p g w y b v k x j q z. There is a word devised to commit some of this list to memory 'Etaoin Shrdlu. There are other common lists however, in Press Reporting, in Religious writings, Scientific writing, General fiction and even on the old system of Morse Code. The first twelve letters in the above list occur in 80% of all words in the English language