--- t.c ---
#define NULL ((void *)0L)
void debug_Error(char *expr, char *fmt, ...)
{
}
#define Error(expr, message...)\
        ((expr) ? 0 : (debug_Error(  #expr, __FILE__, __LINE__,
__PRETTY_FUNCTION__, 1 , ## message, NULL), 1))

int main(int argc, char **argv) {
    Error(0, "URL data has invalid (not hexadecimal) value.");
}
--- end of t.c ---

Compiling this test program gives the warning
t.c:9:61: warning: pasting would not give a valid preprocessing token

Works fine without a warning in GCC 2.95.2 (SuSE 7.0) and EGCS 2.91.66
(RedHat 6.2)

rpm -q gcc
gcc-2.96-54