I had an odd experience with a recent connection at NRT. I flew back on 25 January from IAD-NRT-GUM. I came in from IAD on UA 803. Our flight arrived at NRT about 10 minutes late (mind that I had a 45 minute connection which is the BARE minimum legal connection). When I was about to proceed to security there were several agents handing passengers updated tickets. The agent indicated that I would misconnect and they had re-booked me on the later flight from NRT-GUM (5 hour layover). I told them I could still make the flight but he didn't respond.
I got through security in about 10 minutes and rushed to the gate. I got to the gate about 20 minutes before departure but the boarding had been closed (although the jet bridge was still attached) and the agent wouldn't let me get on even though there was space.
I appreciate that UA re-booked me automatically but it is ridiculous that they didn't allow some of us to try and make the connection. I also don't get why they shut the door so early!!!
Lesson learned is that the 45 minute connection means that if you're flight is slightly late they may automatically re-book you on a later flight.