Using version 2.6 of GSL from ftp://ftp.gnu.org/gnu/gsl/gsl-2.6.tar.gz

OS is Ubuntu 18.04.2 LTS

Hardware is a Dell workstation with a 3.60 GHz Intel i7-4790 and 32 GB
of RAM

Compiler is gcc 7.4.0 with the -w option

When the first argument is zero and third argument is a NaN,
gsl_sf_hyperg_U_int returns 1. When the first argument is nonzero, the
error handler is invoked as would be expected.

#include <gsl/gsl_sf.h>
#include <stdio.h>
#include <gsl/gsl_math.h>

int main (void){
double out;
out = gsl_sf_hyperg_U_int(0, 1, GSL_NAN);

printf("%f\n", out);
printf("%.21e\n", out);
return 0;
}