Using the option --extra-port with the thread-handling=pool-of-threads will cause the server to crash on Mac OSX 10.5 (I haven't tested on any other OS's).

I am able to use the other options without problem (thread-handling=pool-of-threads, thread-pool-size, max-extra-connections). However, using the --extra-port option results in an immediate shutdown. I have tried a wide variety of values for --extra-port with no change in server behavior.

To repeat you can run the test pool_of_threads from the test suite on Mac OSX:
main.pool_of_threads [ fail ]
Test ended at 2009-03-23 18:41:41

CURRENT_TEST: main.pool_of_threads

The server [mysqld.1 - pid: 42686, winpid: 42686, exit: 0] crashed while running 'check testcase before test'

I have attached trace files of a regular server that crashed on startup when using extra-port as well as a trace file of pool_of_threads failing (built with compile-pentium-debug-max)