This tackle is from a mooring that was not inspected or maintained frequently enough. The shackle came open by itself (note the intact zip ties show it did not unscrew). Unfortunately a friend rented this mooring, his boat went aground and was damaged. The party he rented it from is denying responsibility. Unfortunate for everyone but the lawyers. :(

Lesson: Make sure your mooring is inspected at least every 2-3 years. If you rent a mooring make sure you know its maintenance schedule, and IMHO it would be a good idea to verify it with the person who actually does the maintenance.