Facebook announced that it accidentally shared 6 million users’ email addresses or telephone numbers due to a software bug in its system. The complex bug exposed users’ contact information to other users who were connected to them.

The breach was caused by an unfortunate combination of “People You May Know” and “Download Your Information” features. So when some users downloaded their Facebook archives with “Download Your Information,” they may have also saved off contact information they otherwise wouldn’t have had.

Facebook reassures users that “in almost all cases, an email address or telephone number was only exposed to one person.” All affected Facebook users will be notified via email.