The French have highlighted a problem with the double bowline where should you clip and load just one of the loops of the double bowline it is possible to pull through the rope and with catastrophic results.
The BCA have issued the following statement:
Warning – correct use of Bowline on the Bight Knots
BCA’s Training and Equipment & Techniques Committees would like to highlight the importance of ALWAYS clipping a cowstail through BOTH loops of a Bowline on the Bight knot. Although this has been taught for years by BCA instructors, it appears that many cavers are not aware of the importance.
The problem is that in a fall the knot can slip in such a way that the rope going down the pitch can actually run all the way back through the knot. This means that a caver falling at a pitch head with their cowstail clipped into only ONE of the two loops could potentially plummet all the way to the bottom of the pitch. This cannot happen if they are clipped into BOTH loops.
A useful tip is for the rigger to leave an HMS karabiner clipped between the two loops to make it easier for the rest of the party to clip in and it is always worth remembering that two cowstails are preferable to one.
Two different videos are available to demonstrate the problem:
Here’s a video (I’ve tried to add comments/translations as its in French) that highlights the issue