Bad writing doesn't just make you look dumb.

It makes you look "unteachable."  In Michael Skapinker's opinion piece ("If you want a job, learn your it’s and its") in this morning's Financial Times (here), he points out that adults who don't know the difference between "it's" and "its" are demonstrating that they have a learning curve that's too long.  Here's my favorite quote from the article (and don't forget that this opinion piece is from a non-US paper):

Why, then, does mixing up “it’s” and “its” matter when it rarely causes confusion? Because, as Mr[.] Wiens says, if you haven’t learnt the difference by now, what else haven’t you learnt?

So true.  I cringe when people I know can't tell the difference between "it's" and "its", or "your" and "you're", or "there" and "their."  Maybe there's a learning style problem--I hope so, because otherwise I'm looking at people who don't care enough to learn a set of rules that pre-teens are taught.