I've tried this on several computers of the x86_64 variety, all running a currently up-to-date Arch Linux. JACK is speaking to ALSA, and I've tried it with a number of sound cards from built-in laptop cards and M-Audio Delta PCI cards. It's kind of intermittent, but it happens more often than not. I haven't spotted a pattern yet-- so far it's happened on MP3, vorbis and FLAC files of varying bitrates and lengths.
Here's how I recreate it:
1) start playing a file
2) pause the file
3) stop the file using the command line "--stop" switch
At that point, further commands hang, and the curses interface, if open, does not respond to input. MoC continues to appear in other JACK applications (notably qjackctl and patchage) until I kill the moc server. I can restart it without having to restart JACK.
I've managed to get it to hang using the curses interface alone, but it happens with more regularity using command line options.
I started working with the binary package from the Arch repository, but I've recompiled moc to include the debug options. The debug output of an instance, from the start right up until I have to kill it, is available at http://sam.vis.nu/crap/moc.debug/ .
I know it's a weird one, and I only noticed it because my use case for moc is kind of strange, but I thought I should report it.
Thanks!