Broken Protocol Example 
RFC 2082: RIP-2 MD5 Authentication

This protocol seeks to improve RIP security by authenticating packets with a keyed MD5 hash, and part of the mechanism includes a sequence number "to prevent replay attacks":

(5)  An unsigned 32 bit sequence number.  The sequence number MUST be
        non-decreasing for all messages sent with the same Key ID.

   [...]

   The value used in the sequence number is arbitrary, but two
   suggestions are the time of the message's creation or a simple
   message counter.