The matrixes are lubricated with graphite powder, while the rest of the machine is lubricated with oil. Oil and graphite don't mix very well, and produce a slimy substance that slows down movement of the matrixes, causing them to arrive more slowly than other matrixes and to become assembled out of order.

So while the operator did not make any mistake (and in this case the machine was being driven by a repeating loop of punched tape), the mistake was due to the slow matrix.

Type lines produced from a demonstration Linotype machine operating in the Western Minnesota Stream Threshers Reunion (WMSTR) printing building in Rollag, Minnesota, USA.

(This is an example of how a mistake can be an educational event. I don't feel bad about making light of this typesetting production error since the press operators are running the Linotype machine for operational demonstration purposes only, for the general public to observe and discuss, and so perfect operation for actual typesetting application is not a requirement for the demonstration. Most of these type lines get tossed back in the melting pot after demonstration casting.)