I used to have my Apple Calendar's default set to the "Selected Calendar" (last calendar used), which was very convenient. But now, I am syncing with a Google Calendar at my office, which means that I would sometimes, by default, create a calendar in the office calendar, which I could then change to my personal calendar. THAT ISN'T THE PROBLEM. I later realized that by default, the office calendar would send an email to itself, in the Google Calendar's gmail account. But when I changed the the Calendar to my own personal event, the email alert stayed. What that means is that my Apple Mail was automatically sending emails with information about my personal events from MY email account to MY OFFICE'S gmail account, which is accessible to everybody. Awkward.