When a user creates a TCP socket and tries to connect to the socket without explicitly binding the socket to a local address, the connect call implicitly chooses an appropriate local port. When evaluating candidate local ports, the algorithm checks for conflicts with existing ports by doing a lookup in the connection hash table.
In this circumstance, both the IPv4 and IPv6 code look for exact matches in the hash table. However, the IPv4 code goes a step further and checks whether the proposed 4-tuple will match wildcard (e.g. TCP "listen") entries. The IPv6 code has no such check.
The missing wildcard check can cause problems when connecting to a local server. It is possible that the algorithm will choose the same value for the local port as the foreign port uses. This results in a connection with identical source and destination addresses and ports. Needless to say, this does not work well.