Twitter revealed today (for ‘transparency’s sake), that it had inadvertently handed over phone numbers and email addresses used for account security over to third-party marketers in its Tailored Audiences system. The issue was rectified by September 17th, according to the firm.

“Tailored Audiences is a version of an industry-standard product that allows advertisers to target ads to customers based on the advertiser’s own marketing lists (e.g., email addresses or phone numbers they have compiled). Partner Audiences allows advertisers to use the same Tailored Audiences features to target ads to audiences provided by third-party partners,” Twitter explained on Tuesday. “When an advertiser uploaded their marketing list, we may have matched people on Twitter to their list based on the email or phone number the Twitter account holder provided for safety and security purposes. This was an error and we apologize.”

Social media firms have faced scrutiny for accidental data leaks like this one over the past few years, and Twitter’s gaffe is just one more to add to the growing pile.