He had taken a lower case e matrix, cut it out, turned it upside down, and somehow managed to restore the mat so it would cast within a line all right.
At times, when a new operator was working, he would insert this lower case e into the magazine of his machine, leave it for some time, then retrieve it without the operator knowing.
Soon, the galley proofs would come back to the new operator, with a lower case e upside down.
He would be instructed to run out all the lower case e's and cast a line so they could find the culprit.
But, of course, there was no upside down e, leaving the operator wondering what in the hell happened to his machine.