tnt breaks bedrock

when a dispenser fires an arrow at tnt hovering above lava the arrow drops down places the tnt in the block it landed and blows things up. Showcased by JL https://www.youtube.com/watch?v=uzhaHQrhiSE&feature=c4-overview&list=UUWf3wmxbUr5TGmCjHaXV4bQ