I got very annoyed when, a couple of years ago, I started noticing the ubiquity of "then" (when a person meant to write "than") and vice versa, in sentences such as "Canada is bigger then the USA". It just looked so ugly to me and seemed to be a mistake that only the most illiterate could make, but alas no - lots of people sloppily do it all the time, especially on social media.
There's another common native-speaker error which has annoyed me for decades, and it's when some says something like "You should've went to the party - you would've enjoyed it" - or, worse, "You should of went to the party..." Horrible! Grrh!
And a lot of British people (I haven't noticed other native speakers doing this, though I may be mistaken) seem convinced that "aggravation" means "aggression" or "intimidation", whereas, of course, in reality, its true meaning is closer to "exacerbation"
It's almost understandable (though nonetheless unforgivable) that many native English speakers confuse "it's" and "its", but how-oh-how did "Your being pedantic" ever become the g*ddam norm, fer chr*ssakes?! I find these things quite exasperating sometimes.
And I've noticed Spanish people doing some truly awful things with their mother tongue, such as writing "Haber que pasa" instead of "A ver que pasa"; or "Ya lo e echo" for "Ya lo he hecho"; and saying "Habian muchos coches en la calle", when they should, correctly, be saying "Habia muchos coches en la calle".
What about you? What clumsy, lazy, ugly misuses of your own mother tongue have you noticed your fellow native speakers being guilty of? Do you bother to correct people, or are you worried about being branded a pedant?