Main post: viewtopic.php?f=194&t=51751
If a station includes a 180 degree turn (with the train extending to both sides of the turn) the inserters behind the turn are off by 1:
You can still place 2x6 inserters per wagon, but you have to place them one tile to the back of what you would expect visually and of what the yellow placement hint box suggests.
Pretty obscure, so feel free to place in the "wontfix" bin
screnshot of whole station
blueprint