Where it can go Wrong...
It’s vital that the crew in the cockpit know what the person on the foredeck is trying to do. Everyone needs to concentrate and it helps to have someone in charge other than the helm, even if there are only a few crew.
The ropes are often led the wrong way or twisted around another rope or wire. Preparation is the best way to avoid this. The wind has to be very light to allow you to detach a sheet or guy from a hoisted spinnaker while you undo a tangle. Normally you have to drop and start again.
Sometimes the head of the sail fills with wind and there is a twist in the middle creating a wineglass effect. To remove this you have to unravel it upwards. The helm has to steer downwind to blanket the spinnaker, being careful not to gybe. Pull on the sheet, either from the cockpit or side deck, and try and persuade the twist to move upwards. This isn’t going to work if the spinnaker is filling at the top. If it’s really jammed it will have to come down.
If the spinnaker fills on the way up it’s going to be hard work winching the halyard. The helm can help by steering downwind.
There are other problems, such as hoisting it the wrong way up, which means dropping it again and hoping no one notices.