Originally Posted by SquidPuppet

Under the right conditions they can get hot enough that the oil will separate from the thickening agent. It's been said that the oil can run out of the hub and you lose your lube. The Klunker crew that invented mountain biking used to run a downhill race in some CA mountains that was long enough and steep enough that they were on the brakes long and hard enough to cause that problem, forcing them to repack their hubs for each race. Their slang term for the mountain and the race was Repack.