The confused misuse of the following similar sounding words, but with very different uses:
To, too, two
Your, you're
Here, hear
There, their, they're
And so on. It astounds me how often I see these used incorrectly in news stories, posts, print ads, other promotional materials. I usually automatically assume that writer is an amateur (perhaps unfairly). I now believe that there is no real effort put into editing anymore, which used to be a real career profession. Editing is lacking, even in news organizations. I'm not the grammar police, but in professional writing situations, I expect more.
And no, I'm not a teacher, editor, or writer. But these things used to be drilled into the heads of young students and we usually got it right by the 5th grade.