Another yet unknown is manual control or AFCS ( if so equipped ) A/P engaged after t/o. In certain situations and circumstances, such as systems failures or crew errors disengagement of AP and FD may result.
Have a look at AAIB report 6/2013 for G-SYGA on 12/09/12 where the go round mode was inadvertently actived causing a short breakdown in SA and a/c descending below cleared altitude.