Democrats screw up when they accidentally tell the truth (about what they think, causes they support, or how they fell about anyone that disagrees). The quinticential example is the famous Hillary gaffe with Trump voters:
❝ ...you can put half of Trump supporters into what I call the basket of deplorables. Right? Racist, sexist, homophobic, xenophobic, Islamaphobic, you name it. ❞
Here's a brief summary of Hillary's scandals (with links to more on each of them). This isn't meant as a balanced piece to show what good she's accomplished as a politicians or person (that would be a much shorter list), the intent is just to show the pattern of scandals that her detractors recognized and her proponents ignore. If you want the pro-Hillary spin just listen to her, the NYT, CNN or MSNBC, they carry her water for her.
I don't really think think that knowledge makes people humble, and arrogance makes people ignorant: that stuff comes from deeper down. But a little knowledge gives people a bravado that more knowledgable people often don't have (called the Dunning-Kruger effect). And arrogance makes people not question their righteousness. So there's a correlation, even if it's not causal.