Illusion/allusion isn't a grammatical mistake. They're too words that mean different things, but which are very similar syntactically (you can swap between them

Illusion/allusion isn't a grammatical mistake. They're too words that mean different things, but which are very similar syntactically (you can swap between them in a sentence and get two valid sentences which mean entirely different things). This can be a pernicious source of errors of communication. It is good to point it out.