Readers were encouraged to scour the story for the missing "D and C", which was only supposed to appear in one place in the book; here as it turns out:
That should have been a clue that something odd was up, especially since, as it happened, the story was NOT continued on the next page (there was a subscription ad). However, Weisinger didn't look carefully enough at the issue before shipping it to the printers:
Both the word "and" and the word "goodbye" contain a D, although as far as I know, there was no other C in the story.
The results were published in Superman #174:
And when the story was reprinted in Superman #202, the "and" was contracted to "an'", and "goodbye, boss" became "so long, boss".
End of the errors with regard to this story? Not quite. DC reprinted the story again in the book, Superman in the '60s, only this time a production error left the "Continued on Next Page Following" out of the story, so that anybody reading that story would be quite confused, as for example this person:
"The Bizarro Invasion Of Earth" -- also known as "The Great DC Contest", this story invited readers to use their detective skill to find out what made it so unusual. The answer was that the story was crafted to only use the letters D and C once each.
(SPOILERS HO! -- as far as I can tell, the letters only appear together in the opening splash panel which takes place in front of the "City Dump" sign. However, the bottom panel on page 4 has a caption referring to the "Daily Planet." I believe this was said to be an editorial mistake.)
I don't have the Superman in the 1960s book to check this out, but in the original, there are a couple references to "The Planet" without the "Daily" part. And the opening splash panel was specifically mentioned in the rules as not counting.
The usually reliable Fred Hembeck misremembers the situation here.
Of course, the whole carefully conceived plan backfired when, at the very bottom of page 3, the production department had mindlessly pasted in one of their standard "Continued on the next page following" blurbs because, well, the story WAS continued on the next page following, dig? Several issues later, the prize--an autographed Curt Swan cover!!--was awarded not to someone who discovered Mort's strategically hidden letters but who was first to recognize his stupid error!
Actually, as you can see from the announcement in Superman #174, the prizes went to those who discovered the intended "DC" (which was in the "Continued" blurb) and not to those who discovered the mistakenly included two Ds, and there were 25 Swan covers awarded, not just one.
Hat Tip: Commenter Kelly, who pointed this out in a comment on my post about Weisinger's earlier Great Superman Boo-Boo Contest.
Update: Is this story cursed or what? My longtime buddy Snard notes in the comments that there was a D added to the reprint in Superman #202, shown here:
This is probably what the guy who reviewed Superman in the '60s was talking about. In the original it's just "Later, in the parking lot...."