Calendar Spam
My wife and I share calendars, and a few minutes ago she noticed a spammy notice on her calendar. Turns out it was on mine (surprise; my email address is way more public). I found the explanation here; it turns out that anyone can add items to your Google calendar if they know your email address. Look at the settings:
The default (which I just changed to "No" is simply "Yes" - meaning, anyone who knows your email address can throw crap on your calendar - which will then generate alerts sent to you. Meh.