Unallowable Mistakes in English

One of my students once asked me, what mistakes are not allowed in the English language. What would getting on the nerves of native speakers if they hear the English language from, for example, Russian people? I answered, rude grammar mistakes and wrong pronunciation. What can you add? Maybe you can remember some funny situations related to language mistakes. =))

Well, on one level we're actually quite tolerant of mistakes as long as the speaker is making an effort.  We'll try to understand, and even bite our tongues if any laughable mistakes come up.

On the other hand, if the speaker refuses to make an effort, such as choosing simpler forms of grammar when something more specific is needed, then that gets on our nerves. Also, pretending that their mistakes are unimportant (or worse, correct) is rude. It's basically saying, ''I don't care about your language enough to speak it properly.''

I'm trying to think of some funny mistakes, but most of them are related to bad language. ;)