The GPS one is a prime example. The door systems on the modern trains (the ones with sliding doors that don't have to be slammed shut and opened by reaching out of a window and fumbling for a lever) are GPS actuated. These doors will not allow passengers to open them unless the location of the train can automatically be established as being within a few metres of a normal station platform stopping point. The upshot? When it's cloudy or there is any kind of reception fault (as when we get back into London's Victoria station and we're under 20 ft of steel-reinforced concrete) the doors cannot be opened without the driver entering the positioning coordinates manually.
A driver was telling me that there is no 'look just open the bloody doors - I've got a key' button. So journey's all over the south coast are now delayed by really stupid door faults. Ironically the most reliable trains are the slam-door variety I mentioned earlier (which are eminently usable despite feeling like Stephenson's Rocket - unless you are in a wheelchair and then you can pretty much forget it).