As my English is pretty bad, I'm tolerant of other non-native speakers mistakes. I actually often find them interesting. Often you can guess people's first language by the type of mistakes they make, or, even more telling, by grammatically correct, but awkward or unusual sentence structures. I feel it teachs me something about their native language and I find it neat.
I'm also tolerant of mistakes due to disabilities such as dyslexia. It is always the same patterns so once I'm used to it I don't mind too much.
I'm however very, very annoyed at errors that I'd call 'trendy', not that they are on purpose, but rather, that they seem accepted because they are so widespread. Their/they're/There being one of them. Completely offputting.
Also, vocabulary mistakes that change the sense of the sentence entirely, especially when both make sense, so it is impossible to know what the person means. If I read any 'defiantly' instead of definitely, I'll definitely stop reading 