Weak IV
An initialisation vector that leaks information about the key, which made WEP vulnerable to the FMS attack.
In 2001 Fluhrer, Mantin and Shamir showed that the RC4 key schedule reveals the first keystream byte for certain IV values. In WEP the IV was only 24 bits and was sent in the clear, which gave an attacker a channel for recovering key bytes one at a time. The IV space was small and reused often, which made the problem practical.
Modern protocols use nonces or random IVs from a much larger space. The lesson still holds: a cipher is no stronger than its key-setup scheme.