I Knew It - Several of Clancy's theories, such as high ammunition expenditure and attack helicopters being surprisingly vulnerable, were later proven correct during the various conflicts in the Middle East.
The most famous example is the "F-19 Frisbee stealth fighter". Clancy fell hook, line, and sinker for a fake description of the rumored stealth fighter's appearance and capabilities leaked by the Pentagon to conceal the real engineering principles behind the actual F-117 Nighthawk.
NATO at the time had caught wind of a ZSU-30 antiaircraft gun, and had thought it to be a gatling-type antiaircraft gun, in which role it features in the novel. In actuality, the Soviet 30mm antiaircraft gun would be the 9K22 Tunguska—not a rotary-cannon design at all.