Here's one example each in a variety of different categories. None are unique; I just limited myself to one.

Individual abuse: https://www.vice.com/en/article/k7bqew/us-marshal-securus-phone-location-tracked

Targeting people for recrimination extrajudicially (not just cell phones): https://www.huffpost.com/entry/nsa-porn-muslims_n_4346128

Extension of powers beyond what is revealed to the public or legally permitted: https://www.reuters.com/article/usa-security-nsa-idUSL1N0XY1OK20150507

Invading the privacy of foreign leaders who are close allies: https://www.reuters.com/world/europe/us-security-agency-spied-merkel-other-top-european-officials-through-danish-2021-05-30/

And so forth and so on. Also, it's kind of hilarious that you ask for examples while also arguing for the right of the government to keep things secret so we never find out any examples--don't you see the inherent contradiction here?

Also, maybe you forgot Hoover, but the whole "spy on 'enemies of the state' meaning people who threaten my power" thing has a documented history in the United States as well. See COINTELPRO, and note that the harms caused by unjustified suspicion plus collection of circumstantial evidence that could be spun at least enough to create doubt were in many cases not revealed until decades later. (See also McCarthyism.)

A democracy can engage in surveillance in a responsible way, but it has to be done in a way that is highly resistant to abuse or, surprise!, people will abuse it sooner or later. The most essential part of this is to have robust oversight at multiple levels so you don't need to take surveillance as an all-or-nothing proposition; you can continually guide it towards finding actual danger and not, say, surreptitiously gathering material on perceived political opponents until there is enough to publicly disgrace them at will.