False positives in Gmail are rare but not non-zero and there’s always a possibility that an important email may miss your Inbox because Gmail filters have marked that message as spam.
And when you get hundreds of spam messages each day, extracting those legitimate emails from spam gets all the more tough.
There’s some hope as Gmail has included a new "Never Send it to Spam" option in their filter that allows you to set rules for messages that should never be marked as spam.
For instance, if you work for IBM, you could say that any message with "ibm.com" in the To: field should not be classified as spam.
Thus you’ll never miss any important email from your boss. Also see: Gmail Search Commands