A couple of years ago we bought a Topclimber. It 's an excellent device that allows one of us to climb the mast without having to be hoisted up by the other. Unfortunately, the main shackle was inserted in such a way that the friction while climbing was making the pin unscrew.
We learned about this when Alex climbed to get our spinnaker halyard down when our nephew released it by mistake. We didn 't have a spare halyard to use as a backup safety line. Alex noticed at the very top of the mast that the pin was holding by a single thread! It really shook him but he was able to reverse the shackle so that the friction now makes the pin screw itself in rather than out.
Needless to say, we always use a spare halyard as a backup safety line now. We also have a mountaineering harness that we use with the spare halyard in case the Topclimber fails in some unanticipated way.
Please check the shackle if you have a Topclimber. Although the owner has assured us that the shackles are properly aligned, we recently saw one at a boat show that was incorrectly installed.
Vice Commodore, OCC