Two-time pad
Reusing a one-time pad key cancels the key out of the equation and leaves the XOR of the two plaintexts behind.
If C1 = P1 XOR K and C2 = P2 XOR K, then C1 XOR C2 = P1 XOR P2. The key is gone, and what remains is two plaintexts mixed together. Language statistics, guesswork and a known phrase, a crib, can often pull them apart. This happened in practice: the Venona project, in which US signals intelligence began reading Soviet diplomatic and intelligence traffic in 1943, relied heavily on Soviet one-time pad pages that had been reused.