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This is from a YouTube video back in 2012 but it looks like their knot was not correctly tied and unraveled once it was weighted? The video has comments disabled so I can't see what the consensus is.
Is it an incorrectly tied clove hitch?
Still pretty scary to watch!
http://youtu.be/2ey-k5ICWNA
Yikes.....
I tried to freeze frame that video a bit and it looks like the problem was at the anchor and the rope was freely flowing out of the rope bag. Even if they tied the clove hitch poorly, this could have been prevented with the back up knot.
hard to see the actual knot, but will offer a couple of thoughts. the reason an 8 block has largely replaced the biner block using the clove hitch, is that the 8 block is a contingency block that can easily be changed to a lower, if needed. but a significant secondary factor is that the clove hitch can, and does, come undone, spontaneously, when the knot is unloaded, especially with a stiff or fat rope. I had a personal scary experience with this- I tied a good clove hitch on a biner block, and then loaded it snugly against the rap ring, as my sister began a 200 ft rap in Behunin. After stepping over the lip, she thought of something else- she wanted a friend to take a picture, and stepped back up onto the lip, for just a second, to say that, and she then stepped back off the edge. when she stepped up, releasing the tension from the stiff 9mm rope, the clove hitch exploded, coming undone, and then being just three raps around the biner, so when she stepped back over the edge, it started to just pull through. I had left 8 feet of tail on the rope, to tie on the pull rope, and was standing right at the rap station, so I was able to grab it after she had only dropped a foot, but it was a scary moment. shortly after that I switched to an 8 block, which does not change its nature by merely being loaded and unloaded. Many of the hard core biner block community switched to a triple clove, the extra wrap prevents a straight clove from coming undone. the other thought I had looking at this video, was that the rappeller failed to test load the anchor set up on flat ground in a safe position, before stepping over the edge. whatever the anchor or block, it is a good idea to test load it hard, on top, to check that it is secure, before you drop over the edge.