The use of the word "expert" FOR EVERYTHING.

Something I always specify in interviews: being an expert at something, for me, means that I could write a book about it.

So, I prefer to say "I am good at <this>", instead of saying "expert".

The problem is this leads to unclear skill appreciation!!
Like, yes, I spent the last 5 years coding partial differential equations in parallel mainly using C.
Would I call myself an expert in C?
No.
Never.

I am good at it? yes.

So why is this rude?
Because, in case you have sense of honesty and/or the simple inability to call yourself an expert about everything, this can lead to discussion where you have to explain "oh ok, I am not an expert at numerical analysis and solving PDE's but still it was a big part of my last position...so I am probably more of an expert compared to the 22 year old guy that just came out of school who PRETENDS he is an expert at this".

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The almost SYSTEMATICAL use of "C/C++" as if it's a thing.

No, sorry, a thousand times "NO", being a C developer is a thing, being a C++ developer is another thing.
It's possible to be both, but it's FAR from being systematical.

It's as if we would see everywhere "Java/Javascript" -> see the heresy?

So this is not exactly "rude" but terribly annoying having to go into details when answering questions about "C/C++" in interviews: "so, well, I am a good C developer but I am just ok at C++ because, you know? C and C++ are not the same thing AT ALL?"...seeing the sense of confusion in their eyes at that moment is really distressing.

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People from full-HR background trying to pretend they know something about the field because they know the words/keywords.

You know?
Like, I am not a biologist, I know the word "DNA" but would I, in front of a biologist that I am recruiting for some reasons, pretend that I KNOW what DNA is? No, I wouldn't and I shouldn't.
Actually, maybe a good idea could be: -> test the communication skills of somebody: "Explain to me what is <this>" (DNA, for instance, or anything else).

So, no, sorry it's not because you know the words "cloud" or "Python" or "server" that you are an expert and talk like you are one.

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Companies/HR people/Business People trying to oversell the company, particularly going into the "we are so much a good-humane company".
It's "rude" in the sense, that we all know that most of the time it's bullshit. And/or in any case, people claiming that are not the people you're gonna end up working with on a daily basis anyway.

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Coding test for senior positions.
I have more than 10 years of experience.
Two master's degrees in Computer Science.
And a PhD also in Computer Science.

So, Yes. I. Can. Code.

I also know that I am not "the best at coding".
I am not trying to be the best at coding...because I am unable of doing so and also because...I am over 23? (or pick any age that would make sens to you)

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Recruitment process that is solely composed of technical test or if you prefer, when the company is not actually recruiting a human being.

I mean, on a not too big company (below 200 people), doing really advanced stuff (way above the typical software engineering kind of work), one could expect a company would try to NOT ONLY get the matching skills but also the matching personality.