You hash a symmetric key and store the hash on your workstation. You call your friend and verbalize the symmetric key to him or her. Your friend then hashes the key using the exact same hash algorithm that you used (without random salting or anything clever). He or she sends you the hash which when you compare with your version of the hash, you find they are not the same hash. From this you conclude that: a. Your friend must have been on a different operating system which resulted in a slightly different implementation of the algorithm.
b. Your friend must have mis-typed the key when you verbalized it over the phone.
c. Your friend must have decrypted your hash to find the key before re-hashing.
d. Your friend must have double-hashed the key