Non-malleability of One Time Pad Encryption
The Problem

The One-Time Pad offers perfect secrecy. However, it does not protect the integrity or authenticity of the message - An adversary can flip bits of the ciphertext, and the receiver will have no way of detecting the manipulation. 

Consider the case where a single bit is sent encrypted with a One-Time Pad, used to indicate a "yes/no" value as to whether or not Alice wants to have lunch with Bob. If Mallory intercepts the message $C$ and computes $C \oplus 1$ and then sends that value to Bob instead, Bob will receive the opposite of Alice's intended message - and he will have no way of knowing that it has been tampered with.

The solution

Use a MAC to ensure the integrity and authenticity of the message.