In the extreme, this could become a Denial of service attack against all messages on a particular channel using that cipher.
In such a situation, the attacker can turn it into a similar message but one in which some important information is altered.
For example, a change in the destination address might alter the message route in a way that will force re-encryption with a weaker cipher, thus possibly making it easier for an attacker to decipher the message.
[2] When applied to digital signatures, the attacker might be able to change a promissory note stating "I owe you $10.00" into one stating "I owe you $10,000".
A keyed message authentication code, digital signature, or other authentication mechanism allows the recipient to detect if any bits were flipped in transit.