This happened to me a couple of years ago. Flew UA from SEA into NRT, and then was to switch to CX for the hop into TPE.

Got to the CX counter to get my boarding pass, and they would not let me fly because my passport only had five months on it. I had completely overlooked this.

I missed the CX flight, so I went back over to the UA Red Carpet Club, and the helpful CSR in the club called ahead to TPE (I am not sure who she called, maybe immigration?) and got permission for me to fly into TPE (no extra fee), AND rebooked me onto UA metal. Now that's what I call customer service! ^