On crowded platforms, it can be hard for the train driver to see the "Platform Train Interface", particularly on a curved platform. This is what LUL calls an "oblique image" - where people on the platform block the driver's view of the PTI. We'd call it a blind spot.
There are dozens of these all over the Tube, posing a serious safety risk to passengers. Some of the worst locations have been given additional station staff support from the Special Requirements Team so CSAs on the platform can assist drivers. But this isn't a "special" requirement - it's a permanent one!