They were nearly untouchable. One of the A-12s going over North Vietnam took a piece of shrapnel during a mission.
"This time as he approached the capital city, the North Vietnamese fired at least six missiles at him. Looking through his rear-view periscope, Sullivan saw six missile vapor trails climb to about 90,000 feet before converging on his aircraft. He noted the approach of four missiles, and although they all detonated behind him, one came within 100-200 yards of his plane.
He continued the mission and landed safely, but the post-flight inspection crew made a surprising discovery: two small pieces of metal had penetrated the plane’s lower right wing and lodged against the support structure by the wing’s fuel tank. The fragments were not warhead pellets, but may have been part of the debris from one of the missile detonations near the plane."
https://www.cia.gov/news-information/featured-story-archive/2013-featured-story-archive/pieces-of-history-missile-debris-from-a-12-oxcart.html