Re: write-region is mishandling an error

[Richard Stallman]

    Which creates an empty file named "Test at 12" and no error occurs.
    Initially I didn't notice that the filename was truncated.
    Once I did I realized that : is illegal in a file name.
    Some kind of error must be occurring that isn't being reported.

If you run under a debugger with a breakpoint at Fsignal you will see
if any Emacs Lisp error occurs.  I suspect you will find there is none;
I suspect that Windows truncates the file name without complaining.