Allowing the halyard to run through your hands is dangerous - the line may burn your hands and be impossible to grasp. Dexter Holaday, a marine surveyor in Connecticut, could have been seriously injured when a woman who was tailing let go of the line. He plunged toward the deck but someone had the presence of mind to step on the line. Dexter, incidentally, no longer goes aloft when he does a survey. Check the halyard.
Failure can occur sooner in the tropics or on boats that are frequently raced hard.
Conversely, a halyard that is old and frayed can't be relied on. A halyard that was being used to hoist a Florida man aloft broke, according to the surveyor's report, for no other reason than it was old and hadn't been inspected before it was used. The man who was being hoisted bounced off of the boom but, luckily, was not seriously injured. While the entire halyard should be inspected, failure is most likely to occur within a few feet of the shackle, at the point where it rests against the masthead sheave while the sail is under load. Check the condition of the mast itself.