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.
"foo:bar" is a special thingy under WinNT (or maybe under NTFS), kind of
like a "fork" in MacOS speech. It seems it's called "stream".
Maybe Emacs should warn the user that she's about to use a little-known
corner of her OS semantics.