A 1980's film about a computer nerd who's keen on computers, and in his endeavours to find a backdoor into what he thinks is a computer game he ends up hacking into a computer that's in charge of the defense of the country, sparking a full blown alert (as the base staff can't tell whether they're watching a simulation or a real attack).
The boy disconnects, after having his fun, but the war computer keeps playing the game, and rings him back, trying to reach a conclusion (not a theoretical one, mind you, it's now busy cracking the codes to launch real nuclear missiles, from the silos it's recently been given full automatic control over).
The film's rumoured to be based on a real incident.