I've been downloading a lot of MIDI's recently (call it a phase), and I've noticed OS X's surprisingly unstable MIDI playback. Using the Finder's preview
very often results in a crash, and opening a second file in QuickTime player with one MIDI already playing almost always results in a crash (and sometimes just opening one file crashes too, same as the Finder's preview). Perhaps it could be blamed on poorly created files (I would suppose that's possible to do, though I no nothing about it).
Once the files are playing, everything is fine, but I've experienced more QuickTime and Finder crashes from this than under any other circumstances that I can recall (at least I can be thankful that with OS X the crashes don't affect the rest of the system at all!

).