[lmm]

> By this point, any cross-platform file tool that isn't using Unicode as a lowest-common denominator for filenames and similar things to insure maximal compatibility is likely ready to cause havoc.

Be that as it may, Mercurial has existing repositories that may use non-unicode filenames, and just crashing whenever you try to operate on them is probably not an acceptable way forward.

[WorldMaker]

Sure, but that's also not the only resulting option; instead of erroring you could also do something nice like help those users migrate to cleaner Unicode encodings of their filenames by asking them to correct mistakes or provide information about the original encoding. It takes more code to do that than just throwing an error, of course, but who knows how many users that might help that don't even realize why their repositories don't work correctly on, say, Windows.