A more test driven day than usual. Yesterday I noticed a test case was failing on Windows in a way not related to what it was intended to test, and fixed the test case to not fail.. But knew I'd need to get to the bottom of what broke it eventually.
Digging into that today, I eventually (after rather a long time stuck) determined the bug involved automatic conflict resolution, but only happened on systems without symlink support. This let me reproduce it on FAT outside Windows and do some fast TDD iterations in a much less unwieldly environment and fix the bug.