When does fwprintf return a negative number?

I couldn't find it in the docs anywhere, all I did find was that it returns a negative number when an error occurs. What error can it be?

The error occurs in a function that looks like this:

void foo(wchar_t** a)
{
    for (int i = 0; i < N; i++)
        if (fwprintf(stderr, L"%ls ", a[i]) < 0)
            puts("OOPS!");
    fwprintf(stderr, L"\n");
}

Also, this error appears only at one point during the program execution and stays there, as if some kind of limit was reached.