I am not sure that there is one example which can meet all your criteria, though there are many examples of stupid things that (almost) everyone agrees he should not have done:

https://en.wikipedia.org/wiki/WE_Charity_scandal

https://en.wikipedia.org/wiki/SNC-Lavalin_affair

https://en.wikipedia.org/wiki/Aga_Khan_affair

https://www.bbc.com/news/world-asia-india-43151115

I still believe his most egregious betrayal of voters was his abandoning proportional representation, without providing any justification.