Maggin said in Miracle Monday that Superman, realizing or not, could do no mistake.
I think you are referring to the passage where Superman had just defeated C. W. Saturn (
Chapter 23).
Miracle Monday[/i]"]What Superman did not realize—it did not matter whether he realized this or not—was that in this extended instant he could do or say no wrong. There was a right and a wrong in the Universe, and Superman was no more capable of erring here and now than C. W. Saturn was capable of defying him.
Maggin didn't say that Superman never made mistakes, only that in this particular situation he could not. As a matter of fact, in
Chapter 6 Superman did make a mistake. He (as Clark Kent) cued the wrong videotape. Maggin even says that it was "just a touch horrifying" that Superman had made the mistake.
Regarding the topic question, I think there are several examples of Superman's failures in the pre-Crisis comics. But Superman never failed to
try. He persevered. He never quit. He was Superman.
I can enjoy reading about a hero who fails, picks himself up, and tries again. I can't enjoy reading about a hero who fails, mopes about, and gives up.