To help ensure uniqueness of IV's used in a CFB AES-256 cipher, software I know of gathers bits from various sources including the plaintext being enciphered, and runs that through an SHA-256 hash. The resulting 128 bits are used as an IV. I suppose, because CFB is used, this only occurs for the first block.
The plaintext changes over time, and is enciphered many times over its lifetime. An attacker may well have access to multiple ciphertexts of plaintexts which only differ by as little as 1 bit in 4096.
Is this likely to adversely affect the cryptographic strength of the ciphertexts by much?