One of the examples I had in mind relates to what should have been a fairly innocuous incident.
Hurricane Dorian last year threatened serious damage to parts of the Eastern United States, and *has* wrecked the Bahamas to the tune of several billion dollars' worth of damage. So, naturally, as it was progressing through the Atlantic, the eyes of many were on its trail.
So when Trump accidentally suggested that Alabama was threatened, the correct thing for him to do was to apologise for the error, and no harm would have been done. An easy mistake to make, and of course nobody expects Trump to be an expert hurricane forecaster.
What Trump *actually* did was double down on his error, insisting that he was entirely correct, forcing a weather agency to apologise for reporting the actual truth -- ie that Alabama was never in any serious danger at any point in the forecast, doctored an official map in contravention of federal law, and essentially dragged politics into weather-forecasting for no good reason other than his massive ego and his refusal to admit to even the tiniest chance that he had made a mistake. It was disgusting and shameful, and what is even more shameful is that he appears to have forced, directly or indirectly, official weather agencies in the US to go along with it.
And, as I say, the thing is that all Trump needed to do was to admit that he'd "misspoke", or made a mistake, by suggesting Alabama was "most likely" in danger, and no-one would have cared. But as soon as he destroyed the integrity of forecasting, that's the point that shows his unfitness for high office. It's one of the more egregious examples, and should really be utterly uncontroversial.