Whenever I fix some playback bugs in OpenMPT, I usually create a simple test case and reference the filename of that test case in relevant parts of the source code. So far these filenames have only been useful to me, but now I've started creating a list similar to this one (http://schismtracker.org/wiki/Player%20abuse%20tests) which explains all the test cases. Since some of the test cases have been written years ago, they may lack proper descriptions and revision information.
I've started with the XM format (http://wiki.openmpt.org/Development:_Test_Cases/XM), other formats will follow shortly.