Some background on this comic:

Transcript of relevant bits:

In my first year or so of drawing The Far Side, I was scared to death of making mistakes in the artwork. Incredibly, I had never heard of a product called "White Out" (for covering up mistakes) and the smallest screw-up meant starting over. So, as I've indicated under each of these cartoons, I sometimes left things out

The fellow coming through the door is incompletely drawn because I feared the lines of his body would interfere with the auditioner's head. Obviously, I choked.

That's actually quite impressive to miss by THAT much!