When people deny that they have a regional accent and assert that they speak “Standard American English.”
The USA is huge. Regional variations in accent and diction are valid. There is no “correct” American accent. Many words and turns of phrase differ regionally.
Edit: fully aware that SAE exists as a concept. But it lacks: an official dictionary, an official guide to grammar, an official guide to pronunciation, an authority that sets and maintains the standard.
Example of the pet peeve:
Person A (region 1 accent): “I love your accent. Are you from (region 2) originally?”
Person B (region 2 accent): “I was born there, but I don’t have an accent. I speak standard English. Maybe you think I have an accent because you do.”