Where there are convertible seats, the first row of economy remains in Club configuration with no B seat. This is for safety reasons, as the knees of the person sitting in the C seat are pretty invisible to anyone (or anything) coming aft through the curtain.

Unsurprisingly, the first row of economy is therefore prized by many. But the curtain can move, even at a very late stage before the flight. This can mean that you are in the same seat number but no longer in the front row, or it can mean that you get re-seated in a random undesirable seat somewhere else in the aircraft. This causes some people a great deal of angst.