The latest of Elon Musc's attempts at landing a Falcon 9 rocket have failed ("failure" looks pretty spectacular from afar... see the video). It's not a surprise that pulling this off is a tricky piece of engineering.
In a tweet Saturday afternoon, Musk said, “Cause of hard rocket landing confirmed as due to slower than expected throttle valve response.” Musk added that SpaceX will attempt another launch and landing of the reusable Falcon 9 rocket in two months.