I am (or should I say was—since Linotype machines are hard to come by these days) a journeyman Linotype operator. I am offering an additional two cents worth of information about "etaoin shrdlu" (although the actual value of these comments are admittedly subject to the judgment of the reader).
The sequence of letters occasionally appearing in print were not necessarily the result of an error in the previous line. A good keyboardist, as he was assembling a line of matrices and spacebands in preparation for casting, would instinctively know if he had stroked a wrong key. Rather than sending the line with the error through the casting process, then reassembling the line from the beginning, casting the correct line, then (hopefully) remembering to pull out the incorrect line and discard it, he would simply reach up, pull out the incorrect matrix from the assembler, and place it to the right of his copy tray.
Obviously, after accumulating a number of these "mats," he would deem it time to return them to circulation. He could do this by getting up from his chair, going to the back of the Linotype, pulling the second elevator shifter out to clear the second elevator, and sliding the mats onto the second elevator bar to be redistributed into the magazine. Or he could simply place the random mats into the assembler, then fill the line with more random mats and space bands, and send the line through the normal casting and distributing process, taking care, of course, to see that this line, once cast, was pulled from the galley.
Having said all that, the length of line, hence the number of matrices in the line, might determine whether the operator followed the procedure elucidated by mjdixon1 above (e.g., single-column newspaper "straight" matter), or the one just suggested (book composition). While all this explaining may seem somewhat esoteric, if not indeed obscure, to those who are far removed from the wonders of hot-metal linecasting, perhaps one can get a glimmer of the complexity not only of the Linotype machine itself, but also the skill—and artistry—demanded of the competent operator. Vive la Linotype!