Update: Please see the full final report here.
On June 11 2016 (UTC), we started sending an email to all active subscribers who provided an email address, informing them of an update to our subscriber agreement. This was done via an automated system which contained a bug that mistakenly prepended between 0 and 7,618 other email addresses to the body of the email. The result was that recipients could see the email addresses of other recipients. The problem was noticed and the system was stopped after 7,618 out of approximately 383,000 emails (1.9%) were sent. Each email mistakenly contained the email addresses from the emails sent prior to it, so earlier emails contained fewer addresses than later ones.
We take our relationship with our users very seriously and apologize for the error. We will be doing a thorough postmortem to determine exactly how this happened and how we can prevent something like this from happening again. We will update this incident report with our conclusions.
If you received one of these emails we ask that you not post lists of email addresses publicly.