Fredgrav
AP/FD OFF, A/THR ON: SPEED mode is active.
If, inadvertently, it is not in speed (because one pilot pushes the LS button instead of the FD, or does not push it at all when commanded), you can be in deep **** (in approach with the A/THR still in THR IDLE or during a TCAS RA "descend descend" and THR CLB still active, for instance).
This condition is very easy to go unnoticed, believe me. You can have a full FMA and the FPD and just ignore it, when workload is high.
always check and callout "speed" after selecting "FDs OFF". My advise