Deliberate miscalibration of shells in Schindler's List -- did this happen in real life?

As a personal opinion, I'd say he wanted people not to be treated abhorrently, or killed for no reason other than their faith or ethnicity. The war he could do little about; his factory & its workers, he could.
It is estimated that he spent over a million dollars of his own money in efforts towards this end.

All the reports I've ever read about Schindler's List claim it is 'mostly true'. Though it is based on a single book, and some parts may have been elided to give the movie a good story line, the basic facts have been attested to by many of the survivors.

Wikipedia states

Few if any useful artillery shells were produced at the plant. When officials from the Armaments Ministry questioned the factory's low output, Schindler bought finished goods on the black market and resold them as his own.

That the essence of the story is true is attested to by the actions of the State of Israel, in later years

He died on 9 October 1974 and is buried in Jerusalem on Mount Zion, the only member of the Nazi Party to be honoured in this way. For his work during the war, on 8 May 1962, Yad Vashem invited Schindler to a ceremony in which a carob tree was planted in his honor on the Avenue of the Righteous. He and his wife, Emilie, were named Righteous Among the Nations, an award bestowed by the State of Israel on non-Jews who took an active role to rescue Jews during the Holocaust

From comments [Thanks, Paulie_D], an abridged version of a video transcript from a survivor, someone actually on one of Schindler's lists - from The United States Holocaust Museum

Ludmilla worked in a factory at the Plaszow labor camp for a businessman who was a friend of the German industrialist Oskar Schindler. In October 1944, Schindler attempted to save some Jewish workers by relocating them to a munitions factory in Bruennlitz, in the Sudetenland. Ludmilla was among those on Schindler's list to be relocated.

Ludmilla: "in Bruennlitz, he changed to a production of shells, from enamelware. He had to prove really, because they arrested him twice because they didn't think that he was working for the war effort. So we were making shells, and we were supposed to polish these shells. But all the production was faulty because we on purpose were sabotaging this, they were never as they ought to be. Always was something flawed in those shells during our months in Bruennlitz. And this is how he could save us, proving that his factory was indispensable for the war effort."