Writing someone off for that common of a grammatical mistake is pretty ridiculous. I'm in a university environment and people still make that very common, colloquial mistake. All it results in is a few jokes.

There's a huuuugggge different between spoken grammar and written. Speech is far less proper but what matters is that communication occurred. We don't write the way we speak for a reason, but we also don't need to speak exactly how we write.

I have a propensity to create spoonerisms and splice my sentences in weird ways. It's funny and doesn't reflect any lack of intelligence. Our human minds are funny when it comes to speech even when we know better.