Correlation attack
An attack on stream ciphers where the output correlates with a single shift register, so that register can be solved on its own.
In 1985 Siegenthaler showed that a combination generator, where the keystream is a nonlinear function of several LFSRs, can be broken one register at a time. If the output agrees with one register's output often enough, an attacker can guess that register's state from the keystream and check each guess against a short sample.
The attack is practical when the correlation is weak but the registers are short. It is one reason modern stream ciphers rely on nonlinear components rather than just combining several LFSRs. Adding up random-looking outputs does not automatically give you random output.