According to this story on the Beeb, users of Guardian Soulmates have been targeted with spam.
The publisher blamed the third-party provider and human error.
While mistakes can happen, it is the reason why technological and procedural security controls need to be built in, to reduce the likelihood of such errors occurring.
Users of Guardian Soulmates have been targeted with sexually explicit spam emails after their contact information was accidentally exposed on the dating site. Information from users' profiles was included in the spam messages. The Guardian newspaper's publisher, which runs the service, said "human error" was at fault. Guardian News & Media blamed a third-party technology provider for the problem, which has now been fixed.