Length extension attack
An attacker can append data to a signed message and compute a valid signature without knowing the key.
Merkle–Damgård hashes such as MD5, SHA-1 and SHA-256 output their full internal state as the digest. If you sign a message as hash(key || message), anyone who sees the digest can resume the computation from that state and produce a valid hash of the message plus its padding plus an extension. Flickr's API suffered from exactly this in 2009. HMAC avoids it by hashing the key in two separate passes, which breaks the structure that makes the extension possible. A hash is not a MAC, however good it is as a hash.