In reply to zimpara:
How many times have I tied in to both ends of a double rope with a single knot? Once.
And how many times has one of my doubled ropes jammed immovably? Once.
What are the odds of both happening on the same occasion? Very small.
But they did. Left me on the crux of an E1, untying one knot but both ropes, leaving me unroped but hanging on to the unjammed one with my teeth, then retying it singlehandedly. Good job I learnt the trick in the Scouts.