For example, you can’t start files with a space, something a lot of people do to move files to the top of alphabetical lists.
Do people really do that? That would drive me bonkers. Granted, I mainly work from a CLI so I guess it may not be as annoying if you mainly use a GUI.
And also, I did not realize that you could create filenames that contained spaces at the front (or back) on Windows systems. In fact, I'm not sure that's ever been allowed on Windows... has it?
About 15 years ago, I had to migrate an entire Mac OS9 office group (200 systems) to Windows PC. I wrote a Python script to validate filenames and fix them in cases where they contained illegal Windows characters. We would ftp the files up to a Unix system, run the Python filename cleaning script then ftp down to the Windows PCs. It worked rather well, except for cases where the accountants named files something like this, "Summer-Report-07/15/1997.csv"
Mac OS9 file names allowed many characters that Windows did not and OSX may have inherited some or all of these things.
AFAIK, MacOS X also disallows NUL and slash, but IIRC its GUI silently switches slash (Unix path separator) with colon (which IIRC was the MacOS 9 path separator). So, if you were to migrate to OS X instead of to Windows (and if my knowledge is correct) you'd just have to replace the / with a : and the users wouldn't even notice.
Just tried it from the command line and, yes, its quite possible on Windows 8.1.
echo hi > " hello.txt"
I have used '~' to cause files to sort at the top - this seems to be some oddity of the sorting order that Explorer uses, since ~ would sort at the end in ASCIIbetical order - but at least it's an allowed character at the beginning of a name in Explorer, unlike space.
For a cloud file synchronization app, though, I'd expect Microsoft to not let these kinds of restrictions leak down to clients.
I don't know about people doing stuff in file systems these days like that, but it is a simple hack.
Then there was the time another package tried to write out street addresses that were something like 3E123 or maybe it was E456. Apparently the control read them as numbers in scientific format that were just way too big.