For me, it's the very common belief that domestic violence victims are "enablers" or "co-dependent" and that a "normal" victim would just leave. The depths of ignorance embedded in this view make me feel both frustrated and angry.
The belief that financial or career success equals intelligence and hard work.
Looking up to billionaires and taking their advice as if they are brilliant and can be emulated instead of knowing that their circumstances are due in large part to luck and timing and are not repeatable.
Also relatedly, the belief that people who excel at one thing should have sway in other areas. Financiers are not necessarily good politicians. Actors' opinions about medicine should not be influential. I don't think many people would readily admit to believing this or those particular examples, but I do think a lot of people succumb to what I call "the fallacy of being smart."
I'm one of them. I generally consider myself smart and I do know a lot about a few things, but I rarely take expert opinions at face value as if I could somehow do the research better myself. Not the case.