The Toffeemen seemed well set to extend a 13-match unbeaten record and dent United's title ambitions until Pienaar inexplicably stuck out his back leg to impede Ryan Giggs two minutes from the end of a keenly-fought north-west derby.
Having already seen his brilliant opener cancelled out by Tim Cahill, Cristiano Ronaldo was in no mood to let Everton off the hook a second time and promptly sent former United keeper Tim Howard the wrong way to convert his 16th goal of the campaign and give the Red Devils a 2-1 win.
Ronaldo's efforts allowed United to close the gap on Premier League table toppers Arsenal to a single point and left Moyes to bury his head in his hands in total disbelief.
"It was a moment of madness," reflected the Everton manager.
"It was one of those moments you look at and think 'that cannot have happened!'.
"There is no explanation. I have not asked Steven about it - I have told him.
"I cannot hide the fact it was a poor decision but if I say that, I also have to say how well he has done for us over the last two months."
Ronaldo was the only member of United's first-team squad to miss Monday's ill-advised players' party.
Instead, the 22-year-old was picking up third prize in the FIFA world player of the year awards in Zurich.
And the former Sporting Lisbon star showed why he is rated so highly with a sensational opener as he collected Carlos Tevez's square pass, stepped inside Lee Carsley, then thrashed the ball home from 20 yards with his left foot.
"It is very difficult to say how you stop Ronaldo," said Moyes.
"In general play we did quite well today but he just has these moments. You always think you can do better, that you can get closer and tighter.
"But sometimes, when you are tight, he is good enough to get away. They have people who are nerveless and can win you games. They can do things which other players cannot do."
"I don't feel as though we have lost," he said, who confirmed star man Mikel Arteta pulled out due to an overnight sickness bug.
"Manchester United are a great side who cause you problems and I feel as if we played well enough to get something out of the game.
"In the end, we have lost to the champions, to a penalty kick in the last minute. I think that is a sign that Everton are getting closer."