Charles H. Spurgeon on Psalms 120:4

Arrows. Coals of juniper. When the tongue is compared to "arrows", there is a reference (according to the Midrash), to the irrevocableness of the tongue's work. Even the lifted sword may be stayed, but the shot arrow may not. The special point to be drawn out in the mention of "coals of juniper", is the inextinguishableness of such fuel. There is a marvellous story in the Midrash which illustrates this very well. Two men in the desert sat down under a juniper tree, and gathered sticks of it where with they cooked their food. After a year they passed over the same spot where was the dust of what they had burned; and, remarking that it was now twelve months since they had the fire, they walked fearlessly upon the dust, and their feet were burned by the "coals" beneath it, which were still unextinguished. —H.T. Armfield.