Tenor saxophone HORRIBLY out of tune on one note?

I just got a used Yamaha YTS-52 tenor sax. Naturally, after I got it
out, I tried to play it. It sounded fine at first, I went up a Bb
Major scale (no octave key) but the G was really flat. Like, it was a
slightly sharp F#. But once I finished the scale and started going
back down, it was fine. It's really just that one note. Any idea what
could be wrong?

Thanks!