I've been having PCBs made in China for the better part of a decade. Most orders go well, but some do not. Recently, I got an email that stated "It is top trace in your file while it is selected 2 layers in the order list. Could you please confirm it ?"
In times past, this meant that the operator did not see any bottom traces and assumed I left out the layer information. I assured the operator not to worry and that there are only bottom copper pads not traces.
But that wasn't the case.
I received PCBs with soldermask and NO COPPER on the bottom layer. It seems I exported the whole directory and missed the bottom layer file. I even checked it in the Gerber viewer, but because I wasn't watching closely, I identified the soldermask as the copper layer.