English is my native language and I'm from the US, for context!

Ah, I didn't know you were specifically talking about spoken language.

I would like to clarify, I agree with your initial point. If a native speaker makes a grammatical mistake (online through text, or offline/spoken), I've also experienced people just shrugging it off (that's kind of where "it's assumed they know better" came from in my first reply).

To elaborate on my comparative experience with pronunciation differences:
Pronunciation is more likely to be perceived as "faulty" if both speakers are from the same area, but pronounce something differently. E.g., I'd never correct accented English pronunciation from a native speaker (be they from England, New Zealand, or just another part of the US, etc), nor from a non native speaker (so long as the non-native speaker's pronunciation wasn't interfering with understanding, and they weren't trying to "improve" their pronunciation). My example comes from friends who have grown up in the same city but pronounce something differently.

Very interesting conclusion in your last sentence!

Also, I wanted to say - I used your own wording as an example of a small mistake. I hope that didn't come across as me speaking down to you! I was wondering at the time if doing that would come across as rude, but I wasn't sure it would, because it was such an understandable mistake to make... I've enjoyed your thoughts and this conversation, so I just wanted to make sure I didn't come across as having engaged impolitely with you.