I will share with you, my recent experience, on B777 climbing out of FL 187 Cleared for FL250, light weight aircraft, about 60 kts head wind, experiencing mod turbulence, so speed was reduced from 310 kts to turb penetration 270 kts, the airplane had a ROC as a result of speed reduction/head wind/light wieght to 6000ft/min
The airplane went into VNAV ALT Mode while passing FL190, which meant the capture phase began at FL190, the pitch mode of the aircraft (as indicated by FD) focussed on capturing FL250, the thrust lever changed to spd mode, once the speed dropped below 270 kts, the airplane wouldnt pitch down because the pitch att was stuck to capture FL250, the climb power then became less to maintain 270 kts as shown by the downward trend on speed.
The a/c had to be changed to V/S mode, to reduce pitch, straight away, and since this was anticipated, nothing went wrong, but FD went into capture phase because the FMC logic was such, this could have resulted into a complicated situation, so this is one case, where FD taking inputs from FMC in a case like this, would have managed to bring the airplane to low speed buffet before having a mode reversion to VNAV SPD again, which would have taken lot of time to actually recover, considering the airplane trim was in support of nose up while it was in VNAV ALT.