A couple of weeks ago, I was climbing with some tree workers, some were very new to climbing on rope. The least experienced of the group was descending on an 8mm bee-line michoacan, tied to velocity rope. He was descending nice and slow, I was watching him from the ground. He asked me about the speed of his descent which was VERY slow at that point- "Is this speed ok, can I go any faster?". I told him he could go a little faster, that his ungloved hands will let him know if he is going too fast.
He sped up to what I would call a normal descent speed, and stopped about 8' above the ground. As he dangled there next to me, we talked about friction, and heat etc. He decided to descend the rest of the way to the ground but could not. His hitch was completely and utterly locked to his rope. He tried to ascend in order to loosen the cord... not a chance. I put my whole body weight on his hitch trying to break it... nothing! We managed to disconnect him from his system, and belay him down to the ground without issue.
This was very perplexing. I started examining his rope, it was covered in pine pitch, the bee-line was essentially glued in place. The heat from his descent, which was not fast, or even close to being uncontrolled, had liquefied the pitch, and saturated his hitch. The stopping at about 8' and hanging there allowed it to cool, and glue his hitch to his rope, and the coils into one solid mass.
This could have been a real problem, had he been climbing alone, or if he had been at height.
Crazy. Wash your ropes.